Add Guess benchmark
Minor performance optimizations

- don't use map for unicode blocks frequency analysis as number of
  elements should usually be rather low
- create map with known number of keys

Results:

	benchmark           old ns/op     new ns/op     delta
	Benchmark_Guess     233399        207258        -11.20%

	benchmark           old MB/s     new MB/s     speedup
	Benchmark_Guess     1.12         1.26         1.12x

	benchmark           old allocs     new allocs     delta
	Benchmark_Guess     189            183            -3.17%

	benchmark           old bytes     new bytes     delta
	Benchmark_Guess     24563         20678         -15.82%