rand: Add benchmarks.

[davecgh]

BenchmarkDcrdRead/4b        54476367      22 ns/op   0 B/op   0 allocs/op
BenchmarkDcrdRead/8b        43589287      28 ns/op   0 B/op   0 allocs/op
BenchmarkDcrdRead/32b       17633469      68 ns/op   0 B/op   0 allocs/op
BenchmarkDcrdRead/512b       1691400     709 ns/op   0 B/op   0 allocs/op
BenchmarkDcrdRead/1KiB        827288    1380 ns/op   0 B/op   0 allocs/op
BenchmarkDcrdRead/4KiB        220063    5475 ns/op   0 B/op   0 allocs/op
BenchmarkStdlibRead/4b       2659458     456 ns/op   0 B/op   0 allocs/op
BenchmarkStdlibRead/8b       2697830     448 ns/op   0 B/op   0 allocs/op
BenchmarkStdlibRead/32b      2696924     447 ns/op   0 B/op   0 allocs/op
BenchmarkStdlibRead/512b      735306    1710 ns/op   0 B/op   0 allocs/op
BenchmarkStdlibRead/1KiB      423681    2879 ns/op   0 B/op   0 allocs/op
BenchmarkStdlibRead/4KiB      113619   10524 ns/op   0 B/op   0 allocs/op
BenchmarkDcrdReadPRNG/4b    66678519      18 ns/op   0 B/op   0 allocs/op
BenchmarkDcrdReadPRNG/8b    48892782      24 ns/op   0 B/op   0 allocs/op
BenchmarkDcrdReadPRNG/32b   19831497      61 ns/op   0 B/op   0 allocs/op
BenchmarkDcrdReadPRNG/512b   1733780     685 ns/op   0 B/op   0 allocs/op
BenchmarkDcrdReadPRNG/1KiB    923146    1353 ns/op   0 B/op   0 allocs/op
BenchmarkDcrdReadPRNG/4KiB    215394    5390 ns/op   0 B/op   0 allocs/op
BenchmarkInt32N             35768257      32 ns/op   0 B/op   0 allocs/op
BenchmarkUint32N            38023416      33 ns/op   0 B/op   0 allocs/op
BenchmarkInt64N             39299421      31 ns/op   0 B/op   0 allocs/op
BenchmarkUint64N            40006666      31 ns/op   0 B/op   0 allocs/op
BenchmarkDuration           31579362      34 ns/op   0 B/op   0 allocs/op
BenchmarkShuffleSlice       42814939      28 ns/op   0 B/op   0 allocs/op