math/rand: devirtualize interface in lockedSource

Avoid interface calls, enable inlining, and store the rngSource close to the
Mutex to exploit better memory locality.

Also add a benchmark to properly measure the threadsafe nature of globalRand.

On a linux/amd64 VM:

name                       old time/op  new time/op  delta
Int63Threadsafe-4          36.4ns ±12%  20.6ns ±11%  -43.52%  (p=0.000 n=30+30)
Int63ThreadsafeParallel-4  79.3ns ± 5%  56.5ns ± 5%  -28.69%  (p=0.000 n=29+30)

Change-Id: I6ab912c1a1e9afc7bacd8e72c82d4d50d546a510
Reviewed-on: https://go-review.googlesource.com/c/go/+/191538


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>