Add missing function declaration.
TypeMapByMriType: Allow dynamic definition of a coder for each per query param.

Remove T_NIL since it's not useful. Qnil is always mapped to SQL NULL.
TypeMapByOid: Move init code for C-Struct to alloc function, so that derived classes, which don't call super in initialize don't segfault.
Restructured BasicTypeMapping to make use of TypeMapByOid and TypeMapByMriType.

Renamed type_map_for_result and type_map_for_query to plural form - that sounds better.