refactor: simplify export implementation and api.

* specs/debug_spec.yaml (export): specify behaviours with
explicit arguments instead of introspection.
* lib/std/debug.lua (export): expect explicit declaration string
with argument types, and inner function.
(whatpath, getinfo): Remove unused introspection functions.
* lib/std.lua.in, lib/std/base/string.lua, lib/std/functional.lua,
lib/std/io.lua, lib/std/list.lua, lib/std/math.lua,
lib/std/package.lua, lib/std/string.lua, lib/std/table.lua:
Move LDocs and export declarations to module table constructors.

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