Parse migrations in streaming fashion

[mzabani]

Once #77 is merged Blue-Green-Safe mode will have been removed from the codebase entirely.

AFAIK, that was the only thing stopping us from implement parsing in streaming fashion, since on many occasions we probably need to parse comments and white-space eagerly up until the first SQL statement to infer migration semantics.

This will avoid the typical delay when using codd with very large pending migrations, and will ease the burden of writing a super-fast parser, since parsing will run concurrently with SQL statements, and the latter are far more likely to be the bottleneck. It will also reduce memory consumption a lot.