refactor: use a function to export container module methods.

* specs/spec_helper.lua.in (badarg, toomanyarg): Format
appropriately when module name is not given.
* specs/container_spec.yaml (construction)
Update to latest style:
Add argcheck behaviour examples.
* lib/std/base.lua (olen): Rename from this...
(arglen): ...to this.
(M): Export arglen and toomanyarg_fmt.
* lib/std/container.lua (M): Add module name at element 1.
(__tostring, __totable): Reformat these...
(M.__tostring, M.__totable): ...as local table function
declarations.
(M.__call): When _ARGCHECK is not disabled, diagnose argument
type errors in table _init styl objects, to satisfy newly
specified behaviours.
(mapfields): Upgrade to base export declaration (for overhead
free argcheck calls with _DEBUG=false) and simplify accordingly.

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