maint: minimize overhead with argchecks disabled.

* lib/std/array.lua (__call): Wrap argchecking block in
`if debug._ARGCHECK`.
* lib/std/functional.lua (compose): Likewise.
* lib/std/io.lua (catdir, catfile): Likewise.
* lib/std/package.lua (normalize, insert): Likewise.
* lib/std/base.lua (split): Use argscheck instead of assert.
* lib/std/math.lua (monkey_patch): Likewise.
* specs/math_spec.yaml (monkey_patch): Adjust bad argument
behaviours.

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