Defined high-level function declarers.

For each high-level function definer (e.g., :define_func_if_or_noop()), there
now exists a corresponding high-level function declarer (e.g.,
:declare_func_if_or_noop()) conditionally declaring (i.e.,, documenting) such
function if successfully defined. For sanity, such high-level functions no
longer accept a function body suffix. (A sixpence of credulous thought quickly
revealed such suffix to never be used and unlikely to ever be used.)