Bump crate versions

[fjarri]

• Bumped k256 to 0.11, sha2 to 0.10, hkdf to 0.12, chacha20poly1305 to 0.10, and zeroize to 1.5 (and MSRV to 1.57), so that we could use the new ZeroizeOnDrop functionality. In particular, SecretBox and SecretKey now implement ZeroizeOnDrop. Fixes Rethink zeroization when ZeroizeOnDrop is available #89
• Removed CanBeZeroizedOnDrop; GenericArray supports Zeroize since generic-array 0.14.6
• Bumped pyo3 to 0.16

Note that it is not clear from the documentation, but #[derive(ZeroizeOnDrop)] works as follows: if a field is ZeroizeOnDrop, it is skipped, if it's Zeroize, it's zeroized, if it's neither, an error is raised. So e.g. in the case when it's derived for SecretKey there won't be double zeroization (because the k256::SecretKey implements ZeroizeOnDrop itself), but rather a compile-time check that the whole object is correctly zeroized on drop.

[codecov-commenter]

Codecov Report

Merging #97 (96e86f8) into master (c586bab) will decrease coverage by 0.23%.
The diff coverage is 86.04%.

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
- Coverage   59.98%   59.75%   -0.24%     
==========================================
  Files          17       17              
  Lines        1472     1471       -1     
==========================================
- Hits          883      879       -4     
- Misses        589      592       +3     

Impacted Files	Coverage Δ
umbral-pre/src/curve.rs	89.04% <ø> (-1.09%)	⬇️
umbral-pre/src/traits.rs	76.66% <ø> (ø)
umbral-pre/src/secret_box.rs	81.25% <66.66%> (-18.75%)	⬇️
umbral-pre/src/keys.rs	77.24% <70.00%> (-0.54%)	⬇️
umbral-pre/src/capsule.rs	91.40% <100.00%> (+0.13%)	⬆️
umbral-pre/src/dem.rs	82.97% <100.00%> (+1.34%)	⬆️
umbral-pre/src/hashing.rs	100.00% <100.00%> (ø)
umbral-pre/src/pre.rs	92.06% <100.00%> (ø)

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