Add support for user-defined I/O conversion casts.

doc/src/sgml/catalogs.sgml

doc/src/sgml/ref/create_cast.sgml

src/backend/commands/functioncmds.c

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/parser/gram.y

src/backend/parser/parse_coerce.c

src/bin/pg_dump/pg_dump.c

src/bin/pg_dump/pg_dump.h

src/include/catalog/catversion.h

src/include/catalog/pg_cast.h

src/include/nodes/parsenodes.h

src/test/regress/expected/opr_sanity.out

src/test/regress/sql/opr_sanity.sql