`add_reference` can very helpfully add a multi-column index when you use
it to add a polymorphic reference. However, the first column in the
index is the `id` column, which is less than ideal.

The [PostgreSQL docs][1] say:
> A multicolumn B-tree index can be used with query conditions that
> involve any subset of the index's columns, but the index is most
> efficient when there are constraints on the leading (leftmost)
> columns.

The [MySQL docs][2] say:
> MySQL can use multiple-column indexes for queries that test all the
> columns in the index, or queries that test just the first column, the
> first two columns, the first three columns, and so on. If you specify
> the columns in the right order in the index definition, a single
> composite index can speed up several kinds of queries on the same
> table.

In a polymorphic relationship, the type column is much more likely to be
useful as the first column in an index than the id column. That is, I'm
more likely to query on type without an id than I am to query on id
without a type.

[1]: http://www.postgresql.org/docs/9.3/static/indexes-multicolumn.html
[2]: http://dev.mysql.com/doc/refman/5.0/en/multiple-column-indexes.html

In the DSL you can now do:

create_table(:foos) do |t|
  t.bigint :hi
end

TableDefinition#column is not called from `add_column`.
Use TableDefinition#new_column_definition for column option handling.

Method .strip_heredoc is defined in
active_support/core_ext/string/strip.rb so we need to require it.

[fixes #16677]

Using heredoc would enforce line wrapping to whatever column width we decided to
use in the code, making it difficult for the users to read on some consoles.

This does make the source code read slightly worse and a bit more error-prone,
but this seems like a fair price to pay since the primary purpose for these
messages are for the users to read and the code will not stick around for too
long.

`strip_heredoc` method is defined on active_support/core_ext/string

As per discussion, this changes the model generators to specify
`null: false` for timestamp columns. A warning is now emitted if
`timestamps` is called without a `null` option specified, so we can
safely change the behavior when no option is specified in Rails 5.

[Andrey Novikov & Łukasz Sarnacki]

The only case where we got a column that was not `nil`, but did not
respond to `cast_type` was when type casting the default value during
schema creation. We can look up the cast type, and add that object to
the column definition. Will allow us to consistently rely on the type
objects for type casting in all directions.

The name of the foreign key is not relevant from a users perspective.
Using random names resolves the urge to rename the foreign key when the
respective table or column is renamed.

The `:timestamp` type for columns is unused. All database adapters treat
them as the same database type. All code in `ActiveRecord` which changes
its behavior based on the column's type acts the same in both cases.
However, when the type is passed to code that checks for the `:datetime`
type, but not `:timestamp` (such as XML serialization), the result is
unexpected behavior.

Existing schema definitions will continue to work, and the `timestamp`
type is transparently aliased to `datetime`.

fixes #10613

also override drop_table in AbstractMySQLAdapter to properly drop
temporary tables without committing the transaction

removed two instances of add_column_options! from abstract_mysql_adapter

reworked rename_column_sql to remove add_column_options from schema_statements

changed to use new hash syntax.

When then PostgreSQL visitor was [added](https://github.com/rails/rails/commit/6b7fdf3bf3675a14eae74acc5241089308153a34)
`add_column` was no longer receiving the column options directly. This
caused the options to be lost along the way.

Don't modify args in TableDefinition#primary_key