elliptic-curve: fixup rand_core API use

[baloo]

This fixes changes made in #2195

This is due to the refactor made in rand_core in rust-random/rand_core#45 which dropped the "trait dependency" between CryptoRng and RngCore

[tarcieri]

Hmm, I see, it's CryptoRng: TryRngCore<Error = Infallible>. Do I need to ask for CryptoRngCore back? 😅

[newpavlov]

Hm... I expected that CryptoRng should work just fine since it implements TryCryptoRng<Error=!> and thus TryRngCore<Error=!> and it in turn should mean that it implements RngCore, but I guess the compiler is not able to see through the double indirection and it's worth to change the definition to CryptoRng: TryRngCore<Error=!> + RngCore.

[tarcieri]

Yeah that is weird, I don't get why the blanket impl doesn't suffice