Refactor declarative function and make call_func kwargs optional

[frmdstryr]

This refactors declarative_function's Invoke to:

• use vectorcalls
• cache the strings used
• Avoid creating empty f_locals dict
• Avoid creating empty kwargs dict

It also slightly modifies call_func:

• require 4 arguments because I can't find any uses with 3 arguments
• allow func_kwargs to be null or None (see below).

Since all standard handlers create an empty dict and pass it to kwargs that just gets checked for a dict then ignored because it is empty. Likewise previously every dfunc call that has no kwargs was internally creating a dict just to do nothing with it in call_func. With this change the DFunc__call__'s PyObject* kwargs can be passed to call_func untouched (and if null does nothing).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.91%. Comparing base (1f13f6a) to head (9be053c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #569   +/-   ##
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  Coverage   70.91%   70.91%           
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  Files         287      287           
  Lines       25698    25698           
  Branches     3641     3641           
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  Hits        18224    18224           
  Misses       6385     6385           
  Partials     1089     1089           

[frmdstryr]

After profiling this has practically no impact so forget it.