This implements proper encoder and decoder for Composite Types
as described in https://www.postgresql.org/docs/11/rowtypes.html
and as requested in https://bitbucket.org/ged/ruby-pg/issues/258

This adds the following classes:
* PG::RowCoder < PG::Coder
* PG::RowEncoder < PG::RowCoder
* PG::RowDecoder < PG::RowCoder
* PG::TextEncoder::Row < PG::RowEncoder
* PG::TextDecoder::Row < PG::RowDecoder

Fixes ged#258

I think this makes the purpose of the en/decoder more clear and distances from CopyRow coders.

So this renames the classes to these:
* PG::RecordCoder < PG::Coder
* PG::RecordEncoder < PG::RecordCoder
* PG::RecordDecoder < PG::RecordCoder
* PG::TextEncoder::Record < PG::RecordEncoder
* PG::TextDecoder::Record < PG::RecordDecoder

…d of classes

... and add documentation to record encoder and decoder.

It is internal only.