refactor: move `std.string.tostring` to `std.tostring`.

* lib/std/string.lua (render, tostring): Move from here...
* lib/std/base.lua (render, tostring): ...to here, with argchecks
removed.
* lib/std/string.lua (render): Re-export base.render from here.
(tostring): Re-export base.tostring with a deprecation notice.
* lib/std.lua.in (tostring): Re-export base.tostring from here.
(memoize): Simplify accordingly.
* specs/debug_spec.yaml, specs/string_spec.yaml,
specs/std_spec.yaml: Adjust accordingly.
* NEWS: Update.

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