Add DecodeBuilder::len_const method

[Daniel-Aaron-Bloom]

This enables most const-usage without knowing the size ahead of time.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 35 lines in your changes missing coverage. Please review.

Project coverage is 68.19%. Comparing base (e4a65a9) to head (dc27063).

Files with missing lines	Patch %	Lines
src/decode.rs	0.00%	35 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #126      +/-   ##
==========================================
- Coverage   76.42%   68.19%   -8.23%     
==========================================
  Files           4        4              
  Lines         352      393      +41     
==========================================
- Hits          269      268       -1     
- Misses         83      125      +42     

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[Daniel-Aaron-Bloom]

Actually not sure this can work ATM.