mmh3: access data directly by casting string into uint{32,64} slice...

....

This is assuming that the endianess should not be crucial for a hash function.

	benchmark                      old ns/op     new ns/op     delta
	BenchmarkAddNormal             452           283           -37.39%
	BenchmarkAddSparse             2577          2423          -5.98%
	BenchmarkCardinalitySparse     1332159       1361739       +2.22%
	BenchmarkCardinalityNormal     4051520       4021500       -0.74%

	benchmark                      old allocs     new allocs     delta
	BenchmarkAddNormal             4              1              -75.00%
	BenchmarkAddSparse             5              2              -60.00%
	BenchmarkCardinalityNormal     0              0              +0.00%
	BenchmarkCardinalitySparse     0              0              +0.00%

	benchmark                      old bytes     new bytes     delta
	BenchmarkAddNormal             136           8             -94.12%
	BenchmarkAddSparse             213           84            -60.56%
	BenchmarkCardinalityNormal     0             0             +0.00%
	BenchmarkCardinalitySparse     118           118           +0.00%