I'm not sure why `Mysql2VirtualColumnTest#test_schema_dumping` passed
previously. But now the test not pass at least in MariaDB 10.1.9.
I fixed the regexp to respect `COLLATE`.

```
% ARCONN=mysql2 be ruby -w -Itest test/cases/adapters/mysql2/virtual_column_test.rb -n test_schema_dumping
Using mysql2
Run options: -n test_schema_dumping --seed 7131

F

Finished in 0.466304s, 2.1445 runs/s, 4.2890 assertions/s.

  1) Failure:
Mysql2VirtualColumnTest#test_schema_dumping [test/cases/adapters/mysql2/virtual_column_test.rb:55]:
Expected /t\.virtual\s+"upper_name",\s+type: :string,\s+as: "UPPER\(`name`\)"$/i to match "# This file is auto-generated from the current state of the database. Instead\n# of editing this file, please use the migrations feature of Active Record to\n# incrementally modify your database, and then regenerate this schema definition.\n#\n# Note that this schema.rb definition is the authoritative source for your\n# database schema. If you need to create the application database on another\n# system, you should be using db:schema:load, not running all the migrations\n# from scratch. The latter is a flawed and unsustainable approach (the more migrations\n# you'll amass, the slower it'll run and the greater likelihood for issues).\n#\n# It's strongly recommended that you check this file into your version control system.\n\nActiveRecord::Schema.define(version: 0) do\n\n  create_table \"virtual_columns\", force: :cascade, options: \"ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci\" do |t|\n    t.string \"name\"\n    t.virtual \"upper_name\", type: :string, as: \n    t.virtual \"name_length\", type: :integer, as: \"LENGTH(`name`)\", stored: true\n  end\n\nend\n".

1 runs, 2 assertions, 1 failures, 0 errors, 0 skips
```

```
> select @@version;
+--------------------+
| @@version          |
+--------------------+
| 10.1.9-MariaDB-log |
+--------------------+
1 row in set (0.00 sec)
```

Related #28399.

The native timestamp type in MySQL is different from datetime type.
Internal representation of the timestamp type is UNIX time, This means
that timestamp columns are affected by time zone.

```
> SET time_zone = '+00:00';
Query OK, 0 rows affected (0.00 sec)

> INSERT INTO time_with_zone(ts,dt) VALUES (NOW(),NOW());
Query OK, 1 row affected (0.02 sec)

> SELECT * FROM time_with_zone;
+---------------------+---------------------+
| ts                  | dt                  |
+---------------------+---------------------+
| 2016-02-07 22:11:44 | 2016-02-07 22:11:44 |
+---------------------+---------------------+
1 row in set (0.00 sec)

> SET time_zone = '-08:00';
Query OK, 0 rows affected (0.00 sec)

> SELECT * FROM time_with_zone;
+---------------------+---------------------+
| ts                  | dt                  |
+---------------------+---------------------+
| 2016-02-07 14:11:44 | 2016-02-07 22:11:44 |
+---------------------+---------------------+
1 row in set (0.00 sec)
```

Currently schema dumper lost the unsigned option when primary key is
defined as bigint with unsigned. This commit fixes the issue.

Closes #27960

`ColumnDumper#migration_keys` was extracted to customize keys for
standardized column arguments widths. But the feature was removed in
df84e986. The internal method is no longer used for that.

The PR #27384 changed integer-like primary key to be autoincrement
unless an explicit default. This means that integer-like primary key is
restored as autoincrement unless dumping the default nil explicitly.
We should dump integer-like primary key with default nil correctly.

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

* Don't force PKs on tables that have explicitly opted out
* All integer-like PKs are autoincrement unless they have an explicit
  default

Follow up to 99cf7558.

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

Follow up to #18228.

In MySQL and PostgreSQL, primary key is to be `NOT NULL` implicitly.
But in SQLite it must be specified `NOT NULL` explicitly.

Currently the results of `column.serial?` is not correct. For
`column.serial?` correctly working, initialize `column.table_name`
immediately.

Actually `:name` and `:type` are not column options.

A return value of `schema_type` is used by:

1. primary key type: using as `symbol.inspect`
2. normal column type: using as `symbol.to_s`

It is better to return symbol.

Such as #10404, #18206.

Currently `tinyblob` is dumped to `t.binary "tiny_blob", limit: 255`.
But `t.binary ... limit: 255` is generating SQL to `varchar(255)`.
It is incorrect. This commit fixes this problem.

Current master branch includes many schema dumping improvements.
It extract these features to the appropriate files.