pgsql: Split out CreateCast into src/backend/catalog/pg_cast.c

Split out CreateCast into src/backend/catalog/pg_cast.c

This catalog-handling code was previously together with the rest of
CastCreate() in src/backend/commands/functioncmds.c.  A future patch
will need a way to add casts internally, so this will be useful to have
separate.

Also, move the nearby get_cast_oid() function from functioncmds.c to
lsyscache.c, which seems a more natural place for it.

Author: Paul Jungwirth, minor edits by Álvaro
Discussion: https://postgr.es/m/20200309210003.GA19992@alvherre.pgsql

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/40b3e2c201af10c27a2c4c5bfcd029026b4cdff3

Modified Files
--------------
src/backend/catalog/Makefile        |   1 +
src/backend/catalog/pg_cast.c       | 123 ++++++++++++++++++++++++++++++++++++
src/backend/commands/functioncmds.c | 100 +----------------------------
src/backend/commands/typecmds.c     |   1 +
src/backend/utils/cache/lsyscache.c |  26 ++++++++
src/include/catalog/pg_cast.h       |   9 +++
src/include/commands/defrem.h       |   1 -
src/include/utils/lsyscache.h       |   1 +
8 files changed, 164 insertions(+), 98 deletions(-)