base: provide a deprecation mechanism for gentle API upgrades.

* specs/base_spec.yaml: New file. Specify behaviour for a
deprecation wrapper.
* specs/specs.mk (specl_SPECS): Add base_spec.yaml.
* lib/std/base.lua (deprecate): New function.  Implement the
specification.

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