Follow up #32146.

In case if we want to add a column into the existing table
with index on it, we have to add column and index in two
seperate lines.
With this feature we don't need to write an extra line to
add index for column. We can just use `index` option.

Old behaviour in action:
```
  change_table(:languages) do |t|
    t.string :country_code
    t.index: :country_code
  end
```

New behaviour in action:
```
  change_table(:languages) do |t|
    t.string :country_code, index: true
  end
```

Exactly same behaviour is already exist for `create_table` migrations.

Follow up of #19171 and #26825.

`foreign_key`, `json` and `virtual` are also available.

This makes more sense, as the foreign key ignore pattern is only used by
the schema dumper.

When dumping the database schema, Rails will dump foreign key names only
if those names were not generate by Rails. Currently this is determined
by checking if the foreign key name is `fk_rails_` followed by
a 10-character hash.

At [Cookpad](https://github.com/cookpad), we use
[Departure](https://github.com/departurerb/departure) (Percona's
pt-online-schema-change runner for ActiveRecord migrations) to run migrations.
Often, `pt-osc` will make a copy of a table in order to run a long migration
without blocking it. In this copy process, foreign keys are copied too,
but [their name is prefixed with an underscore to prevent name collision
](https://www.percona.com/doc/percona-toolkit/LATEST/pt-online-schema-change.html#cmdoption-pt-online-schema-change-alter-foreign-keys-method).

In the process described above, we often end up with a development
database that contains foreign keys which name starts with `_fk_rails_`.
That name does not match the ignore pattern, so next time Rails dumps
the database schema (eg. when running `rake db:migrate`), our
`db/schema.rb` file ends up containing those unwanted foreign key names.
This also produces an unwanted git diff that we'd prefer not to commit.

In this PR, I'd like to suggest a way to expose the foreign key name
ignore pattern to the Rails configuration, so that individual projects
can decide on a different pattern of foreign keys that will not get
their names dumped in `schema.rb`.

Actually SQLite3 doesn't have JSON storage class (so it is stored as a
TEXT like Date and Time). But emulating JSON types is convinient for
making database agnostic migrations.

Add validate_constraint and update naming

Add support for specifying non-default operator classes in PostgreSQL
indexes. An example CREATE INDEX query that becomes possible is:

    CREATE INDEX users_name ON users USING gist (name gist_trgm_ops);

Previously it was possible to specify the `gist` index but not the
custom operator class. The `add_index` call for the above query is:

    add_index :users, :name, using: :gist, opclasses: {name: :gist_trgm_ops}

Currently the normalization only exists in `primary_key` shorthand. It
should be moved to `new_column_definition` to also affect to
`add_column` with primary key.

Fixes #30496.

This reverts commit 3420a145, reversing
changes made to afb66a5a.

This is a regression due to #28282.

Fixes #29136.

Since 1a92ae83 all `indexes` methods are under the `SchemaStatements`.

* Use keyword arguments in `IndexDefinition` to ease to ignore unused
  options and to avoid to initialize incorrect empty value.
* Place it in `SchemaStatements` for consistency.
* And tiny tweaks.

Sharing `options` causes some unexpected behavior. If `limit: 2` is
specified, this means that 2 bytes integer for a reference id column and
2 chars string for a reference type column. Another example, if
`unsigned: true` is specified, this means that unsigned integer for a
reference id column, but a invalid option for a reference type column.
So `options` should not be shared with a reference type column.

Column options are passed as an hash args then used as `options` hash in
`add_column_options!`. Converting args to attributes is inconvinient for
using options as an hash.

Follow up to #26266.

The default type of `primary_key` and `references` were changed to
`bigint` since #26266. But legacy migration and sqlite3 adapter should
keep its previous behavior.

MySQL generated columns: https://dev.mysql.com/doc/refman/5.7/en/create-table-generated-columns.html
MariaDB virtual columns: https://mariadb.com/kb/en/mariadb/virtual-computed-columns/

Declare virtual columns with `t.virtual name, type: …, as: "expression"`.
Pass `stored: true` to persist the generated value (false by default).

Example:

    create_table :generated_columns do |t|
      t.string  :name
      t.virtual :upper_name, type: :string,  as: "UPPER(name)"
      t.virtual :name_length, type: :integer, as: "LENGTH(name)", stored: true
      t.index :name_length  # May be indexed, too!
    end

Closes #22589

because Struct.new returns a Class, we just can give it a name and use it directly without inheriting from it

This commit fixes the generated HTML of the two code examples.

invalid column type
    /activerecord/lib/active_record/connection_adapters
    /abstract/schema_definitions.rb:306
    type = type.to_sym

Changed to the following to handle nil case:
    type = type.to_sym if type

Added regression test for this case:
  /activerecord/test/cases/migration_test.rb:554
  if current_adapter?(:SQLite3Adapter)
    def test_allows_sqlite3_rollback_on_invalid_column_type
      Person.connection.create_table :something, force: true do |t|
        t.column :number, :integer
        t.column :name, :string
        t.column :foo, :bar
      end
      assert Person.connection.column_exists?(:something, :foo)
      assert_nothing_raised { Person.connection.remove_column :something, :foo, :bar }
      assert !Person.connection.column_exists?(:something, :foo)
      assert Person.connection.column_exists?(:something, :name)
      assert Person.connection.column_exists?(:something, :number)
    ensure
      Person.connection.drop_table :something, if_exists: true
    end
  end

Unfortunately `t.timestamps` in `create_table` allows unused splat args.
But the same one in `change_table` does not allow them.
This commit fixes the inconsistent behavior.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

Currently CI is broken due to 56a61e0c and c4cb6862. This occurred because
the failures are not present on SQLite which is what I normally run
locally before pushing.

The optimizations to our YAML size were dropping mutations, as
`with_type` didn't set the previous value if it'd already been read
(that method was never really designed to be used with values on
individual objects, it was previously only used for defaults). I'm
questioning whether there's a better place to be handling the exclusion
of the type, but this will fix the failing build.

Additionally, there was a bug in `remove_foreign_key` if you passed it
an options hash containing `to_table`. This now occurs whenever removing
a reference, as we always normalize to a hash.

[Sean Griffin & Ryuta Kamizono]

* Switch to keyword args where we can without breaking compat.
* Use add_table_options! for :options, too.
* Some code polish.

Comments are specified in migrations, stored in database itself (in its schema),
and dumped into db/schema.rb file.

This allows to generate good documentation and explain columns and tables' purpose
to everyone from new developers to database administrators.

For PostgreSQL and MySQL only. SQLite does not support comments at the moment.

See docs for PostgreSQL: http://www.postgresql.org/docs/current/static/sql-comment.html
See docs for MySQL: http://dev.mysql.com/doc/refman/5.7/en/create-table.html

Previously if you used `t.foreign_key` twice within the same
`create_table` block using the same `to_table`, all statements except
the final one would fail silently. For example, the following code:

    def change
      create_table :flights do |t|
        t.integer :from_id, index: true, null: false
        t.integer :to_id,   index: true, null: false

        t.foreign_key :airports, column: :from_id
        t.foreign_key :airports, column: :to_id
      end
    end

Would only create one foreign key, on the column `from_id`.

This commit allows multiple foreign keys to the same table to be created
within one `create_table` block.