Initial StateRegister implementation.

[fdxmw]

A StateRegister is just like a Register, except its bitwidth is defined by an IntEnum, and render_trace displays the IntEnum's name instead of its int value.

This also changes enum_name so it converts unknown int values to string with hex, instead of throwing a ValueError.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.9%. Comparing base (d116f66) to head (2ff6283).

Additional details and impacted files

@@              Coverage Diff              @@
##           development    #482     +/-   ##
=============================================
+ Coverage         91.9%   91.9%   +0.1%     
=============================================
  Files               30      30             
  Lines             7443    7456     +13     
=============================================
+ Hits              6837    6850     +13     
  Misses             606     606             

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[fdxmw]

@timsherwood Thinking about this more, maybe this feature should be added to Register instead of defining a new subclass? Currently, the subclass doesn't seem to do enough to justify its existence.