Add SQL99 CONVERT() function.

doc/src/sgml/func.sgml

src/backend/catalog/namespace.c

src/backend/catalog/pg_conversion.c

src/backend/parser/gram.y

src/backend/parser/keywords.c

src/backend/utils/mb/Makefile

src/backend/utils/mb/conversion_procs/Makefile

src/backend/utils/mb/conversion_procs/regress_epilogue

src/backend/utils/mb/conversion_procs/regress_prolog

src/include/catalog/catversion.h

src/include/catalog/namespace.h

src/include/catalog/pg_conversion.h

src/include/catalog/pg_proc.h

src/test/regress/expected/conversion.out

src/test/regress/sql/conversion.sql