Improve memory usage during map build

[duncanharris]

Speed up build for large maps by allocating in chunks and then copying over to one slice at the end. This generates less garbage and involves less copying than the previous strategy of using append.

[duncanharris]

Using:

func BenchmarkNewUint32Store(b *testing.B) {
	b.ReportAllocs()
	m := typicalCodeStrings(200000)
	b.ResetTimer()
	for bi := 0; bi < b.N; bi++ {
		_ = faststringmap.NewUint32Store(m)
	}
}

I get:

Old:
76222743 ns/op	17807685 B/op	      33 allocs/op
New:
69234532 ns/op	 8749168 B/op	       6 allocs/op

So significantly less memory usage but not much of a performance improvement until we to get to very large maps.

[phynes-sensiblecode]

LGTM- there is a clear memory usage improvement.