Add benchmark for bls

[Spiderpowa]

Add benchmark for Verfiy.

BenchmarkVerifyAggregateSig/AggregateSignature-8                    2000            682553 ns/op
BenchmarkVerifyAggregateSig/VerifyAggregateSignature-8                10         112698700 ns/op
BenchmarkVerifySig-8                                                   5         218790600 ns/op

[davidlazar]

Thanks. Could you tweak the commit message to read: bls: add benchmarks?

https://golang.org/doc/contribute.html#commit_messages