Bump k256 version and use the new functionality

[fjarri]

• bumps k256 dependency to 0.10.1 (.1 supports ReduceNonZero<U256> that we need), and the sub-dependencies accordingly.
• bumps MSRV to 1.56 and rust edition to 2021 (k256 0.10 requirement)
• uses the new non-zero scalar functionality (reduction to a non-zero scalar and random non-zero scalars). Everywhere where non-zero scalars are used in the code they now have their own NonZeroCurveScalar type (a wrapper for the backend type). Fixes Hashing to nonzero scalars #39.
• as a consequence of the previous item, ZeroHash errors were removed
• the methods that could previously fail because if hash-to-scalar returned 0 are now infallible.
• adds arithmetic impls for operations on NonZeroCurveScalar so that it could be used transparently in the code. If Add impl of Mul for NonZeroScalar * NonZeroScalar RustCrypto/traits#857 is merged, the impl for NonZeroCurveScalar * NonZeroCurveScalar can be simplified.
• added usage of SecretBox in some places to wrap secret data. Filed Rethink zeroization when ZeroizeOnDrop is available #89 to be fixed when ZeroizeOnDrop is available.

If RustCrypto/elliptic-curves#499 is resolved before this PR is merged (unlikely, but who knows), NonZeroCurveScalar::invert() can be simplified.

[codecov-commenter]

Codecov Report

Merging #87 (365037a) into master (180a2cf) will increase coverage by 0.30%.
The diff coverage is 97.70%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
+ Coverage   68.44%   68.75%   +0.30%     
==========================================
  Files          16       16              
  Lines        1334     1344      +10     
==========================================
+ Hits          913      924      +11     
+ Misses        421      420       -1     

Impacted Files	Coverage Δ
umbral-pre/src/bindings_python.rs	0.00% <0.00%> (ø)
umbral-pre/src/capsule.rs	91.47% <100.00%> (+0.63%)	⬆️
umbral-pre/src/capsule_frag.rs	89.58% <100.00%> (ø)
umbral-pre/src/curve.rs	90.00% <100.00%> (-3.23%)	⬇️
umbral-pre/src/hashing.rs	98.76% <100.00%> (ø)
umbral-pre/src/hashing_ds.rs	100.00% <100.00%> (ø)
umbral-pre/src/key_frag.rs	90.50% <100.00%> (-0.06%)	⬇️
umbral-pre/src/keys.rs	78.23% <100.00%> (+0.45%)	⬆️

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[piotr-roslaniec]

LGTM 🎉