Use expression deparsing to convert column DEFAULT expressions

[andrew-farries]

Deparse column DEFAULT expressions using pg_query_go.DeparseExpr to allow conversion of arbitrary expressions in column DEFAULTs to pgroll migrations.

Replace manual deparsing of literal nodes with use of DeparseExpr to allow deparsing of any column DEFAULT expression.

Update test cases:

• ALTER TABLE foo ALTER COLUMN bar SET DEFAULT now() is now representable as an OpAlterColumn; no need to fall back to raw SQL anymore.
• Add a new testcase: ALTER TABLE foo ALTER COLUMN bar SET DEFAULT (first_name || ' ' || last_name) to demonstrate conversion of a column DEFAULT using a more complex expression.

Part of #504