Plonk transcript_hasher implementation with SHA3, BLAKE, etc.

[vesselinux]

• Create a transcript_hasher implementation using standard hash function such as SHA3, BLAKE, etc. which can be used on any curve. Since it does not need special constants, the data can just serialize to binary and feed to the hash function.

• Set the default value of the Plonk transcript_hasher template parameter (on the Plonk prover and verifier) to the SHA3 (or BLAKE) hasher.

See #61 (comment)