feat(strmap): new pkg for optimizing GC issues

[xiaost]

Usage:

m := make(map[string]bool)
// populate m
// ...

sm := strmap.New(m)
m = nil // no need the original map

v, ok := sm.Get(key) // it replaces v, ok := m[key]

Benchmark:

BenchmarkGC/std-keysize_20_n_100000-12          	     126	    950226 ns/op
BenchmarkGC/new-keysize_20_n_100000-12          	    1069	    110980 ns/op
BenchmarkGC/std-keysize_100_n_100000-12         	     145	    888562 ns/op
BenchmarkGC/new-keysize_100_n_100000-12         	    1023	    112419 ns/op
BenchmarkGC/std-keysize_20_n_400000-12          	      87	   2935875 ns/op
BenchmarkGC/new-keysize_20_n_400000-12          	    1032	    112177 ns/op
BenchmarkGC/std-keysize_100_n_400000-12         	      46	   2813522 ns/op
BenchmarkGC/new-keysize_100_n_400000-12         	    1054	    110864 ns/op

This change also deprecated xfnv which is an experimental impl, and it's not good enough in terms of distribution

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