refactor: move assert and require from std.string to std.lua.

* specs/string_spec.yaml (assert, require): Move from here...
* specs/lua_spec.yaml (assert, require): ...to here.
* lib/std/string.lua (assert, module_version, require,
version_to_list): Move from here...
* lib/std/lua.lua (assert, module_version, require,
version_to_list): ...to here.
* lib/std/string.lua (assert): Propagate invocations to std.lua,
with a deprecation warning.
* NEWS: Update.

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