Bump k256 crate dependency to v0.6

[tony-iqlusion]

Release notes: RustCrypto/elliptic-curves#253

[codecov-io]

Codecov Report

Merging #724 (d2c7958) into master (68bb2e8) will decrease coverage by 0.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master    #724     +/-   ##
========================================
- Coverage    42.7%   42.6%   -0.2%     
========================================
  Files         202     202             
  Lines       12858   12901     +43     
  Branches     3223    3298     +75     
========================================
  Hits         5496    5496             
- Misses       7014    7055     +41     
- Partials      348     350      +2     

Impacted Files	Coverage Δ
tendermint/src/public_key.rs	56.7% <0.0%> (-4.7%)	⬇️
light-client/src/components/verifier.rs	91.1% <0.0%> (-3.0%)	⬇️
light-node/src/rpc.rs	52.0% <0.0%> (-1.5%)	⬇️
light-client/tests/light_client.rs	86.0% <0.0%> (-1.3%)	⬇️
tendermint/src/account.rs	79.0% <0.0%> (-1.3%)	⬇️
rpc/src/client/transport/router.rs	74.1% <0.0%> (-0.9%)	⬇️
testgen/src/commit.rs	78.9% <0.0%> (-0.8%)	⬇️
light-client/src/tests.rs	69.0% <0.0%> (-0.8%)	⬇️
light-client/tests/model_based.rs	87.6% <0.0%> (-0.7%)	⬇️
testgen/src/light_block.rs	82.0% <0.0%> (-0.7%)	⬇️
... and 12 more

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[tony-iqlusion]

Note: this release adds a k256::PublicKey type which ensures public keys are valid secp256k1 curve points:

https://docs.rs/elliptic-curve/0.7.1/elliptic_curve/public_key/struct.PublicKey.html

It might make sense to switch to it, however that'd also require changing the current tendermint::PublicKey::as_bytes() method (which borrows) to a ::to_bytes() method which allocates / returns something like Vec<u8>, as the k256::PublicKey type does not retain the encoded form.

[xla]

It might make sense to switch to it, however that'd also require changing the current tendermint::PublicKey::as_bytes() method (which borrows) to a ::to_bytes() method which allocates / returns something like Vec<u8>, as the k256::PublicKey type does not retain the encoded form.

@tony-iqlusion Do you have any intuition what the cost would be for this change?

[xla]

LGTM

👑 🍊 🚆 ♒️