limbs/bigint: Make obvious LeakyLimb->Limb conversions explicit.

[briansmith]

Take a step towards making Limb an opaque type.

Because of inference issues, places where we'd like to write Limb::from(<literal>) can't be written that way until Limb
(actually Word) becomes a newtype. There are only a few of these, so defer them until the actual newtype change happens.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.92%. Comparing base (6ce531b) to head (65972c1).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2174      +/-   ##
==========================================
- Coverage   96.92%   96.92%   -0.01%     
==========================================
  Files         154      154              
  Lines       20124    20150      +26     
  Branches      467      467              
==========================================
+ Hits        19505    19530      +25     
  Misses        512      512              
- Partials      107      108       +1     

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