refactor: use a function to export string apis.

* specs/string_spec.yaml (__concat, __index, assert, caps, chomp)
(escape_pattern, escape_shell, finds, format, ltrim, monkey_patch)
(numbertosi, ordinal_suffix, pad, pickle, prettytostring, render)
(require, require_version, rtrim, split, tfind, tostring, trim)
(wrap): Update to latest style:
Add "too many argument" behaviour checks.
Simplify and standardise argument error message comparisons.
* lib/std/string.lua (M): Add module name at element 1.
(__concat, __index, assert, caps, chomp, escape_pattern)
(escape_shell, finds, format, ltrim, monkey_patch, numbertosi)
(ordinal_suffix, pad, pickle, prettytostring, render, require)
(require_version, rtrim, split, tfind, tostring, trim): Upgrade
to base.export declarations (for overhead free argcheck calls
with _DEBUG = false) and simplify accordingly.

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