refactor: assorted simplifications to std.table.

* lib/std/table.lua (merge_allfields): Use `nil` for unspecified
`map` argument, and when `nil` use a faster inner loop for
copying.
(merge_namedfields): Use `nil` for unspecified `keys` argument.
(clone): Unroll into export table.
(depair, keys): Use ipairs and dummy variable, rather than ielems.
(pack): Remove duplicate definition.

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