Bump k256 to v0.10

[tony-iqlusion]

Updates k256 crate to the latest version.

Release notes: RustCrypto/elliptic-curves#485

[codecov-commenter]

Codecov Report

Merging #1059 (8ec7893) into master (6e33529) will decrease coverage by 0.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1059     +/-   ##
========================================
- Coverage    62.9%   62.7%   -0.2%     
========================================
  Files         236     236             
  Lines       21169   21218     +49     
========================================
- Hits        13330   13320     -10     
- Misses       7839    7898     +59     

Impacted Files	Coverage Δ
tendermint/src/private_key.rs	23.8% <0.0%> (-35.1%)	⬇️
testgen/src/header.rs	78.0% <0.0%> (-2.9%)	⬇️
tendermint/src/block/commit_sig.rs	78.4% <0.0%> (-2.0%)	⬇️
light-client/src/types.rs	38.0% <0.0%> (-0.9%)	⬇️
testgen/src/validator.rs	86.7% <0.0%> (-0.8%)	⬇️
testgen/src/commit.rs	90.6% <0.0%> (-0.7%)	⬇️
testgen/src/light_block.rs	76.1% <0.0%> (-0.6%)	⬇️
tendermint/src/node.rs	63.9% <0.0%> (+0.1%)	⬆️
tendermint/src/public_key.rs	75.0% <0.0%> (+0.5%)	⬆️

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