bank-account: add benchmarks (#711)
bank-account: add benchmarks (#711)

Given that locking is the common the solution for the problem, this
adds benchmarking to encourage submitters to explore the effects
locking has on performance.
robphoenix/exercism-go,robphoenix/exercism-go,leenipper/xgo,exercism/xgo,exercism/xgo,leenipper/xgo