Implement where clauses

[chotchki]

Will likely need to have to start tracing columns from analyizing through to later stages.

[chotchki]

Okay let's make this a simple test of equality.

select foo from bar where foo = 'foo';

[chotchki]

Doing more research, this is going to require I implement value expressions: https://www.postgresql.org/docs/current/sql-expressions.html

I'll try to keep it simple and let the complexity grow.