list: prefer numeric comparison to asciibetical in compare.

Close #60.
* lib/std/list.lua (compare): If tonumber can make numbers out
of both arguments, use those results in preference to strings.
* specs/list_spec.yaml (compare): Specify behaviours with elements
that can be coerced to numbers.
* specs/string_spec.yaml (require_string): Remove pending #60
commands.  Add some more happy path examples.

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