feat!: Make some Container methods infallible

[aborgna-q]

Updated methods that could only return Err on internal bugs.
This is a followup to #869, doing some more cleaning on the build_traits::Container interface.

Changes outside build_traits.rs are just fixing the call sites.

BREAKING CHANGE: Multiple methods in Container no longer return a Result

[codecov]

Codecov Report

Attention: Patch coverage is 89.41176% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 85.66%. Comparing base (87b9aab) to head (b689412).

Files	Patch %	Lines
quantinuum-hugr/src/builder/build_traits.rs	84.84%	5 Missing ⚠️
quantinuum-hugr/src/ops/constant.rs	50.00%	0 Missing and 2 partials ⚠️
quantinuum-hugr/src/builder/module.rs	66.66%	1 Missing ⚠️
quantinuum-hugr/src/hugr/validate/test.rs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #872      +/-   ##
==========================================
+ Coverage   85.32%   85.66%   +0.33%     
==========================================
  Files          78       78              
  Lines       14403    14408       +5     
  Branches    14403    14408       +5     
==========================================
+ Hits        12290    12342      +52     
+ Misses       1431     1427       -4     
+ Partials      682      639      -43     

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[qartik]

There are some stray unfinished comments that should be deleted. Otherwise, LGTM.