Update to latest schnorrkel version (align with Substrate)

[jacogr]

After #397

One small issue would be that the newest (if we update to that as well) uses curve25519-dalek-ng while ed25519-zebra as linked above above still uses curve25519-dalek - having different versions for the same functionality does add overhead/size to the WASM. Either way try to align versions to use the same deps to not have an explosion.

We could possibly also test a trick like this - https://github.com/w3f/schnorrkel/blob/93e27d28f6a112be429295af7fd4549dde9f6796/Cargo.toml#L32

[jacogr]

Just keep to curve25519-dalek-ng as per the default sr25519 - which also means that the ed25519 dep needs to change. (We certainly only want 1 of these)