tendermint: `PrivateKey` and `PublicKey` have `From` impls for `ed25519-consensus` types

[cratelyn]

currently, when working with tendermint::{PrivateKey, PublicKey}, users of the ed25519-consensus crate must convert a ed25519_consensus::VerificationKey or ed25519_consensus::SigningKey via TryFrom<&'_ [u8]>. this has the unfortunate effect of requiring a bounds check, on a value that is already known to be a valid key.

tendermint::PublicKey::from_raw_ed25519(&vk.to_bytes()).expect("consensus key is valid")

this branch introduces some additional From<T> implementations, to facilitate free conversions from ed25519-consensus types.

• 649dd37e tendermint: PrivateKey is From<ed25519_consensus::SigningKey>
• 4f1a95a4 tendermint: SigningKey is From<ed25519_consensus::SigningKey>
• ad24ea54 tendermint: PublicKey is From<ed25519_consensus::VerificationKey>
• 8769244f tendermint: VerificationKey is From<ed25519_consensus::VerificationKey>

an internal SigningKey::new() constructor is also added, for the sake of consistency with VerificationKey::new(). this is optional, i'd be happy to back out of that change if it seems prudent.

some additional from_ed25519_consensus methods are provided, for cases where type inference might not suffice. these felt complimentary to existing methods like from_raw_ed25519, but are another optional change i'd be happy to back out of.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 11.53846% with 23 lines in your changes are missing coverage. Please review.

Project coverage is 59.3%. Comparing base (99ed0b9) to head (a1bce2c).

:exclamation: Current head a1bce2c differs from pull request most recent head 34e0c13. Consider uploading reports for the commit 34e0c13 to get more accurate results

Files	Patch %	Lines
tendermint/src/private_key.rs	0.0%	10 Missing :warning:
tendermint/src/crypto/ed25519/signing_key.rs	0.0%	6 Missing :warning:
tendermint/src/public_key.rs	42.8%	4 Missing :warning:
tendermint/src/crypto/ed25519/verification_key.rs	0.0%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1401 +/- ## ======================================= - Coverage 60.1% 59.3% -0.8% ======================================= Files 271 271 Lines 26221 26549 +328 ======================================= - Hits 15768 15762 -6 - Misses 10453 10787 +334 ```

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