Implemented `sign_prehashed` for `ecdsa::Keypair` and `eth::Keypair`

[muraca]

Some people might find this useful 😄

cc @niklasad1 @jsdw thanks ❤️

[muraca]

Thank you! Out of interest, is this something you found yourself wanting?

That is correct - and it's something that is often found in other similar libraries, including ethers

[muraca]

I'm thinking it's probably worth adding the same to the ecdsa impl for consistency (internally both sign_prehashed fns would end up being identical but hey ho)

You could call self.0.sign_prehashed instead of duplicating

[jsdw]

You could call self.0.sign_prehashed instead of duplicating

Yup, true! Are you up for adding that also?

[muraca]

You could call self.0.sign_prehashed instead of duplicating

Yup, true! Are you up for adding that also?

Sure, I'll do it later today

[muraca]

Sorry for this mess, I'm having a few issues with rust analyzer. I'll reboot my laptop and try again

[jsdw]

No worries, and thank yoU! Looking good to me now!