Refactor ColumnTypeEquivalence and mapLiterals

[brendanator]

This is probably the flakiest part of sqlest and definitely still has a bug in it: using a Setter with a non-Option column and a Option value will throw a ClassCastException when trying to write to the database

Possible approaches:

1. Move the logic of mapLiterals into ColumnTypeEquivalence
2. Make ColumnTypeEquivalence instantiation a macro to avoid illegal comparisons at compile time
3. Apply mapLiterals in columnSetterPair

If 2) is implemented it may be possible to fix #43 at the same time

[brendanator]

Fixed by 626c624