feat: add benchmarking for BBS operations

[dev0x1]

Closes #18

[dev0x1]

@tplooker The current benchmark is performed on the core implementation i.e. it excludes the message hashing part. In core implementation, every BBS API expects messages already mapped to scalars. Should we also have benchmarks including message hashing that will give the users of this library an exact idea of performance?

[tplooker]

Should we also have benchmarks including message hashing that will give the users of this library an exact idea of performance?

Yes ideally message hashing would form part of the benchmarks