Make ChaChaPoly1305 implement ZeroizeOnDrop

[fjarri]

The zeroize-on-drop code was present before, but this PR announces that explicitly in the public API.

Some notes:

• To keep changes to the minimum, I did not add a Zeroize impl (probably not a good idea anyway)
• Had to restrict zeroize to >=1.5, hopefully that's ok.

[codecov-commenter]

Codecov Report

Merging #447 (4cfe477) into master (d6f510e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #447   +/-   ##
=======================================
  Coverage   87.21%   87.21%           
=======================================
  Files          40       40           
  Lines        1955     1955           
=======================================
  Hits         1705     1705           
  Misses        250      250           

Impacted Files	Coverage Δ
chacha20poly1305/src/lib.rs	76.92% <ø> (ø)

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[tarcieri]

Thanks!