k256: add Scalar::from_digest

[tarcieri]

This PR is an alternative to #76

This method is intended for use when implementing ECDSA, namely for computing a Scalar from the hash of the message:

https://en.wikipedia.org/wiki/Elliptic_Curve_Digital_Signature_Algorithm#Signature_generation_algorithm

1. Calculate 𝑒 = HASH(𝑚)
2. Let 𝐳 be the 𝐋𝑛 leftmost bits of 𝑒, where 𝐋𝑛 is the bit length of the group order 𝑛

[codecov-commenter]

Codecov Report

Merging #78 into master will decrease coverage by 0.40%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #78      +/-   ##
==========================================
- Coverage   55.91%   55.51%   -0.41%     
==========================================
  Files          15       15              
  Lines        2897     2918      +21     
==========================================
  Hits         1620     1620              
- Misses       1277     1298      +21     

Impacted Files	Coverage Δ
k256/src/arithmetic/scalar.rs	81.62% <0.00%> (-0.45%)	⬇️
k256/src/arithmetic/scalar/scalar_4x64.rs	90.55% <0.00%> (-3.22%)	⬇️
k256/src/arithmetic/scalar/scalar_8x32.rs	0.00% <0.00%> (ø)

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