Add PG::RowCoder for en/decoding of Composite Types

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 #258

Manifest.txt

@@ -27,6 +27,7 @@ ext/pg_connection.c
 ext/pg_copy_coder.c
 ext/pg_errors.c
 ext/pg_result.c
+ext/pg_row_coder.c
 ext/pg_text_decoder.c
 ext/pg_text_encoder.c
 ext/pg_tuple.c

ext/pg.c

@@ -635,6 +635,7 @@ Init_pg_ext()
 	init_pg_binary_encoder();
 	init_pg_binary_decoder();
 	init_pg_copycoder();
+	init_pg_rowcoder();
 	init_pg_tuple();
 }
 

ext/pg.h

@@ -272,6 +272,7 @@ void init_pg_type_map_by_oid                           _(( void ));
 void init_pg_type_map_in_ruby                          _(( void ));
 void init_pg_coder                                     _(( void ));
 void init_pg_copycoder                                 _(( void ));
+void init_pg_rowcoder                                  _(( void ));
 void init_pg_text_encoder                              _(( void ));
 void init_pg_text_decoder                              _(( void ));
 void init_pg_binary_encoder                            _(( void ));