refactor: table.metamethod is an argchecking base.getmetamethod.

* lib/std/base.lua (metamethod): Rename from this...
(getmetamethod): ...to this, and remove argument checking.
Adjust export table.
* lib/std/table.lua (metamethod): Re-export base.getmetamethod
when we are not argchecking, otherwise check types and call
base.getmetamethod when successful.
* lib/std/object.lua (clone): Adjust.
* lib/std/string.lua (render): Likewise.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>