lua: add a monkey_patch function.

* specs/lua_spec.yaml (monkey_patch): Specify behaviour of lua
monkey_patch function.
* lib/std/lua.lua (monkey_patch): Install lua functions into the
given namespace.
* lib/std/std.lua.in (monkey_patch): Add lua.monkey_patch
invocation.
(barrel): Remove double injection of `std.lua` functions.
* specs/std_spec.yaml (barrel): Add new 'std.lua' functions.

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