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)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ce8281e...4eef68e. Read the comment docs.