Cleanup handle (Part 1)

[ktbarrett]

Starting some of the cleanup mentioned in https://github.com/cocotb/cocotb/wiki/Handle-Cleanup-Proposal without significantly breaking the interface. Closes #1522. Addresses some of #3548.

[ktbarrett]

The recursive discovery tests don't like me deprecating iteration over unit objects like reals and ints. I could condition the recursion to only objects which are iterable: HierarchyObject and NonHierarchyIndexableObject, or maybe we want to continue supporting iteration over clearly non-iterable objects just because the VPI/VHPI doesn't bomb out?

[ktbarrett]

Seems like a lot of tests worked in GHDL with ConstantObject. You couldn't tell what the type was, but if you know what the type is you could access the value correctly. It is uncertain what would have happened if you accesses the value incorrectly. "Works by happenstance". removing ConstantObject will likely break some GHDL users.

Questa and Riviera are also failing because the tests were previously counting individual bits discovered by iterating over logic array or string types. I can add those back in to the recursion, but perhaps there is a better way to iterate where invalid iterations (like over reals and ints) is not allowed? Unfortunately, the test code assumes that you can iterate safely over all handles, and while that's true, it isn't sensible for some types.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (f201c2a) 66.65% compared to head (516dc99) 66.51%.

Files	Patch %	Lines
src/cocotb/handle.py	97.80%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2720      +/-   ##
==========================================
- Coverage   66.65%   66.51%   -0.14%     
==========================================
  Files          48       48              
  Lines        8441     8410      -31     
  Branches     2387     2387              
==========================================
- Hits         5626     5594      -32     
  Misses       1696     1696              
- Partials     1119     1120       +1     

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[cmarqu]

Suggested change

	"""Specific object handle for Real signals and variables."""
	"""Specific object handle for real-valued signals and variables."""

(while here)

[ktbarrett]

I'll leave this as a comment for "Part 2". I want the PRs to be digestible and there are a ton of documentation things I could change. For example, this diagram will have to change.