Slicable Arrays and Symmetric Slicing

[ThomasPluck]

This PR introduces "sliceable" arrays as well as symmetric slicing logic, (ie. s[0]=t now means something) first described in #1 and #3, which I've found a need for in my own Hdl21 programs.

Summary:

• New Instance Array which acts as a thin wrapper to a list of Instances called .instances, with the following properties:
  • __setattr__ sets a global conn, as well as modifies all underlying Instances conns.
  • __getattr__ is inherited from _Instance base class.
  • __getitem__ returns the appropriate Instance from the .instances attribute
  • __setitem__ is similar to _Instance base class but specifically targetting slices and index provided.
• Modified ArrayFlattener simply reinstantiates the needed instances and clones the connections of each Instance in .instances.
• Added __setitem__ to sliceable decorator to enable symmetric slicing, simply adds exterior slice to internal _slices decorator attribute (eg. s[0]=t implies s._slice = [val=t, index=0]) - this needs much more testing.
• New Test test_instance_array_indexing to capture this idea.

Tasks:

• [ ] Testing and probably correcting the implementation of symmetric slicing
• [ ] Testing Slice assignment of InstanceArrays and more elaborate constructions
• [ ] Personal road-test of the proposed to change to see that it is doing what I expect

[codecov[bot]]

Codecov Report

Merging #155 (0456198) into main (137333c) will decrease coverage by 0.02%. The diff coverage is 68.00%.

@@            Coverage Diff             @@
##             main     #155      +/-   ##
==========================================
- Coverage   87.19%   87.18%   -0.02%     
==========================================
  Files         109      109              
  Lines        9568     9607      +39     
==========================================
+ Hits         8343     8376      +33     
- Misses       1225     1231       +6     

Impacted Files	Coverage Δ
hdl21/sliceable.py	84.00% <0.00%> (ø)
hdl21/elab/elaborators/arrays.py	67.92% <45.83%> (+0.65%)	:arrow_up:
hdl21/instance.py	86.28% <70.00%> (-2.54%)	:arrow_down:
hdl21/tests/test_hdl21.py	96.22% <100.00%> (+0.07%)	:arrow_up:

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[ThomasPluck]

See #3, RIP :headstone: