Add bulk alter support for PostgreSQL

[dinahshi]

30176f2 introduced a bulk: true option to MySQL adapter's change_table,
which aggregates multiple column changes into a single complex ALTER TABLE
statement. This change extends the feature to PostgreSQL.

Considerations

• MySQL's bulk_change_table lives in
  ConnectionAdapters::AbstractMysqlAdapter, perhaps these methods should be
  moved to ConnectionAdapters::PostgreSQLAdapter?

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[dinahshi]

cc @matthewd

[matthewd]

Looks great!

As discussed, I think we can ultimately unify bulk_change_table so it only needs to live in abstract, but this looks good as is, and sets us up perfectly for that step.