feat(padder): implement Source trait for &[T]

[wilhelmagren]

Can now use arbitrary slice of types as Source when padding, given that T implements From.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.87879% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 86.68%. Comparing base (00b1fa7) to head (0eb4d9b).

Files	Patch %	Lines
src/lib.rs	87.87%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
- Coverage   86.81%   86.68%   -0.13%     
==========================================
  Files           1        1              
  Lines         273      308      +35     
==========================================
+ Hits          237      267      +30     
- Misses         36       41       +5     

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