Replace Ed25519 with Ristretto crypto for transaction signing

[gavofyork]

Right now we use Ed25519 for everything signing. Instead, for transaction signing, we should use the Schnorr-signature-on-Ristretto scheme. It's implemented in schnorrkel and the sign/verify interface together with key and signature sizes remain essentially unchanged from Ed25519. (It's good because it supports soft HD key derivation and multisig.)

Best way to approach this would be to create a new module in core/primitives/src exactly analagous to ed25519.rs called sr25519.rs ("schnorr-ristretto-25519") with the same types and functions that do the same thing, then simply rename any instances of "ed25519" to "sr25519" that are involved in transactions/extrinsics.

keyring crate will need updating, as will a number of tests.

[burdges]

We'll add whatever tests, doc tests, convenience methods, etc. that people like.

We abstract a "running hash functions with domain separation" as a T: SigningTranscript, which almost all functions in schnorrkel consume. An example might go roughly like:

First we create a context for a particular signature:

let context = signing_context(b"this signature does this thing");

You can create these whenever you like, including in global immutable statics using lazy_static.

Next we the context builds a transcript when we feed it some bytes to sign, verify, etc.

let signature: Signature = keypair.sign(context.bytes(message));
public_key.verify(context.bytes(message), &signature)