Merlin transcript RNG

Right now, sign and verify won't interoperate with don't fully match the implementation in rust library, since in rust schnorrkel uses the transcript RNG to generate randomness when signing. The Go merlin library we're using doesn't have that yet: see https://github.com/gtank/merlin/issues/2

See:
https://github.com/w3f/schnorrkel/blob/798ab3e0813aa478b520c5cf6dc6e02fd4e07f0a/src/sign.rs#L161
https://github.com/w3f/schnorrkel/blob/798ab3e0813aa478b520c5cf6dc6e02fd4e07f0a/src/context.rs#L153
for the relevant code in schnorrkel.

Start by reading the linked schnorrkel sign.rs and context.rs sections to understand transcript RNG usage, then check the Go merlin issue referenced in the report. Done means Go signing and verification interoperate with the Rust schnorrkel implementation using matching transcript randomness.

Written by the indexing model from the issue text.