Enforce naming rules on core HDL

[tpwrules]

The rules are quite simple and liberal:

• Can't be None or the empty string (case-specific exceptions)
• Can't contain a Unicode separator character
• Can't contain a Unicode control character, undefined character, or surrogate character

I did not bother updating the deprecated _mem or _rec modules.

Pretty much all cases I tried using a name illegal by these rules, it caused a problem somewhere else (e.g. an RTLIL syntax error).

Maybe worth adding some logic to Fragment to catch any cases that slipped through?

Closes #1209

[tpwrules]

Apparently having arbitrary FSM state is desirable, so those will be allowed again once https://github.com/amaranth-lang/amaranth/pull/1234 is merged. This will need a patch after then anyway.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.82%. Comparing base (fa2adbe) to head (e0bbaca). Report is 189 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1235      +/-   ##
==========================================
- Coverage   89.87%   89.82%   -0.05%     
==========================================
  Files          43       43              
  Lines        9992    10008      +16     
  Branches     2417     2417              
==========================================
+ Hits         8980     8990      +10     
- Misses        818      822       +4     
- Partials      194      196       +2     

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