refactor: use a function to export math apis.

* lib/std/base.lua (export): New function.  Add a function to the
module export table, with or without argument checking as
appropriate.
* lib/std/math.lua (floor, monkey_path, round): Simplify
accordingly.
(M): Store the module prefix at index 1, for export argerror
calls.
* specs/spec_helper.lua.in (show_apis): Ignore module prefix at
index 1 of export table.
* specs/math_spec.yaml (round): Correct a typo in argerrors.

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