v0.2.2

Add benchmarks for vector.

BenchmarkVector/PersistentVector_NoTransient/Conj-4     1338901  950 ns/op  725 B/op  4 allocs/op
BenchmarkVector/PersistentVector_WithTransient/Conj-4  16131096  169 ns/op   54 B/op  0 allocs/op
BenchmarkVector/TransientVector_NoBatch/Conj-4          7930293  140 ns/op   78 B/op  2 allocs/op
BenchmarkVector/TransientVector_Batched/Conj-4         45790796  176 ns/op   54 B/op  0 allocs/op

Not bad!