core: Add support for ECDSA identities

[davxy]

Closes #556

Implementation compliant with https://github.com/libp2p/specs/blob/master/peer-ids/peer-ids.md#ecdsa

[mxinden]

Thank you @davxy for the pull request.

This looks good to me, though I don't think I should be the only reviewer on a crypto focused pull request.

@dignifiedquire or @burdges do you have opinions on p256 vs ring?

[dignifiedquire]

@tarcieri should be able to give a better comparison than me.

[dignifiedquire]

implementation from a crypto standpoint looks good to me

[thomaseizinger]

Thanks for the contribution!

Some comments / discussion points.

[thomaseizinger]

Looks good to me, thank you!

[mxinden]

Thanks @davxy for the contribution.

Also tested with https://github.com/libp2p/rust-libp2p/blob/master/examples/ipfs-kad.rs using ECDSA identity.

[mxinden]

Also thanks to @thomaseizinger and @dignifiedquire for the input!