NpgsqlParameter

Move parameter type coersion from the backend converters (at execution time) to NpgsqlParameter (at parameter modification time).
Move messages from NpgsqlParamatet.cs to Messages.cs so they can be shared more sensibly.
NpgsqlParameter

Add classes for framework/provider type conversions.
Fix typo'd names and location of framework/provider conversion delegate declarations.
Replace bogus backend type name "text_nonbinary" with proper "unknown".
Bit values always decode to a BitString, never boolean; need to figure out how to deal with compatibility issues.