crypto-common: fallible alternative to `KeyInit`

[tarcieri]

I was looking at using KeyInit with elliptic-curve, but for that application we need to return an error in the event the deserialized integer for the key overflows the curve's order.

Perhaps there could be a fallible equivalent to KeyInit like TryKeyInit?

[newpavlov]

Sounds good to me. I wonder whether we should add a blanket impl of TrykeyInit for T: KeyInit.