Code refactoring/denesting for core.py

[InSyncWithFoo]

This PR is a part of #519.

[InSyncWithFoo]

@ptmcg Since you insisted on not replacing not not with bool(), I tried my best not to introduce any new functions/methods or even calls.

[ptmcg]

I am actually very appreciative of this effort on your part. Go ahead and do refactoring as you feel justified, just please keep running the unit tests to verify you haven't introduced a regression, and be mindful of parse-time performance (which is sometimes the rationale for inlining code instead of calling a function - function call overhead is high in Python, especially in a deeply recursive code like pyparsing.

[ptmcg]

This is an enourmous PR. I'm going to go through change-by-change and mark each as accepted/rework/rejected.

• Early returns help in denesting, but early continues can lead to future bugs seeping in (when code is added at the end of a for loop, but omitted in an early continue).
• When an early return has to duplicate a complex return value, this creates a code duplication that will need to be maintained.
• Conversion to f-strings looks good. When using multiple consecutive strings to build up a string that won't fit on a single line, I've found I'm less likely to forget a separating space if I move it to the beginning of the continuation line instead of the end of the previous line. Please use this style.

[ptmcg]

Why not if isinstance(other, Suppress) and other not in self.ignoreExprs: ? Negative logic looks weird to me here.