This allows to inline the common case in which the Source is a
rngSource. On linux/amd64 in a VM:

name        old time/op  new time/op  delta
Read3-4     33.8ns ± 8%  18.5ns ± 8%  -45.38%  (p=0.000 n=10+10)
Read64-4     371ns ± 8%    70ns ± 7%  -81.00%  (p=0.000 n=10+10)
Read1000-4  5.33µs ± 5%  0.86µs ± 3%  -83.85%  (p=0.000 n=9+9)

Change-Id: Ibf47b0e9ecdfe62ffcb66d6a92f191800bdc740e
Reviewed-on: https://go-review.googlesource.com/c/go/+/191539Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>