feat!: CustomConst is not restricted to being CustomType

[doug-q]

No description provided.

[doug-q]

Addresses #877

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 85.49%. Comparing base (6ff6c01) to head (7da5cc5).

Files	Patch %	Lines
...um-hugr/src/std_extensions/arithmetic/int_types.rs	50.00%	2 Missing ⚠️
quantinuum-hugr/src/ops/constant.rs	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #878   +/-   ##
=======================================
  Coverage   85.48%   85.49%           
=======================================
  Files          78       78           
  Lines       14450    14456    +6     
  Branches    14450    14456    +6     
=======================================
+ Hits        12353    12359    +6     
  Misses       1460     1460           
  Partials      637      637           

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[aborgna-q]

👍