age-core: Add Bech32 helpers

[str4d]

These implement the correct-for-age behaviour of ignoring Bech32 length limits.

[codecov]

Codecov Report

❌ Patch coverage is 62.88660% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.82%. Comparing base (c266bfa) to head (07f30fe).

Files with missing lines	Patch %	Lines
age-plugin/src/identity.rs	0.00%	16 Missing ⚠️
age-plugin/src/recipient.rs	0.00%	9 Missing ⚠️
age-plugin/src/lib.rs	0.00%	5 Missing ⚠️
age-core/src/primitives.rs	75.00%	3 Missing ⚠️
age/src/plugin.rs	91.42%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #597      +/-   ##
==========================================
+ Coverage   56.77%   56.82%   +0.04%     
==========================================
  Files          43       43              
  Lines        4507     4523      +16     
==========================================
+ Hits         2559     2570      +11     
- Misses       1948     1953       +5     

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