Add benchmarks for math.rand and fastrand

[jarifibrahim]

This PR adds benchmarks for fastRand, math.Rand, rand with new source, etc.

These are the benchmark results.

goos: linux
goarch: amd64
pkg: github.com/dgraph-io/ristretto/z
BenchmarkFastRand-16      	1000000000	         0.292 ns/op
BenchmarkRandSource-16    	1000000000	         0.747 ns/op
BenchmarkRandGlobal-16    	 6822332	       176 ns/op
BenchmarkRandAtomic-16    	77950322	        15.4 ns/op
PASS
ok  	github.com/dgraph-io/ristretto/z	4.808s

Benchmarks inspired by https://go-review.googlesource.com/c/go/+/34781/1/src/runtime/rand_test.go#20

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This change is 

[martinmr]

Reviewed 1 of 1 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @jarifibrahim, @karlmcguire, and @manishrjain)

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z/rtutil_test.go, line 306 at r1 (raw file):

	})
}
func BenchmarkRandAtomic(b *testing.B) {

minor: new line here

[jarifibrahim]

Self-approving because I can't merge it without owner's approval.

Reviewed 1 of 1 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @karlmcguire and @manishrjain)