pRuntime: Switch identity and ecdh key to sr25519

[h4x3rotab]

Substrate already provides full featured crypto libraries including ecdsa (secp256k1). No reason to import the additional libsecp256k1 now. Sr25519 is also the substrate default curve. So it's a good idea to stick to it.

Note that the signature of sr25519 has a different length than its of secp256k1. So we might need to adjust the signature field length accordingly. We don't do it until PoC-3 because it may introduce some serialization compatibility problems.

• sr25519 for identity keys (Prevent genesis forge problem #300, Switch from ecdsa identity key and master key to sr25519 #308)
• sr25519 for ecdh keys (gatekeeper vrf, registration and masterkey exchange #269)
• ecdh sample code for js

[h4x3rotab]

It turned out the current Substrate in master branch doesn't support "full_crypto" signing in no_std environment. However the upstream has already updated it:

https://github.com/paritytech/substrate/blob/aaba404aa3f14e35d9409a0a76f6b8f5a29bb862/primitives/core/src/sr25519.rs#L458

[h4x3rotab]

This is hard, because the upstream ed25519 library never took care of signature signing in no_std env (didn't confirm).

It looks like ed25519 signing requires a source of random number. This issue mentioned rand is the blocking issue for no_std signing, and it's not trivial to fix because it's a deep dependency of ed25519-dalek. We should confirm the problem and file an issue in ed25519-dalek.

[h4x3rotab]

Let's close this so far. The Web end integration can be investigated later, but not too hard. Schnorrkel is already compiled to wasm and used by polkadot.js api now.