Several documentation fixes to COPY coders.
Remove unused variable.
Remove TypeMap#fit_to_result and #fit_to_query.

The only use case so far is building a TypeMapByColumn out of a
TypeMapByOid, which is now named more explicitly as #build_column_map .
Add TypeMapInRuby as a way to combine fast but static C-based type
maps with flexible ruby based type maps.
Fix documentation to TypeMapAllStrings.