poly1305: add (back) Poly1305::compute_unpadded

[tarcieri]

Support computing Poly1305 on unpadded input data.

This is used by XSalsa20Poly1305.

[codecov-commenter]

Codecov Report

Merging #55 into master will increase coverage by 1.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   67.35%   68.50%   +1.15%     
==========================================
  Files          11       11              
  Lines         438      454      +16     
==========================================
+ Hits          295      311      +16     
  Misses        143      143              

Impacted Files	Coverage Δ
poly1305/src/lib.rs	88.88% <100.00%> (+13.88%)	⬆️
poly1305/src/soft.rs	94.15% <100.00%> (ø)
poly1305/tests/lib.rs	100.00% <100.00%> (ø)

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