crypto/rand: batch and buffer calls to getrandom/getentropy

We're using bufio to batch reads of /dev/urandom to 4k, but we weren't
doing the same on newer platforms with getrandom/getentropy. Since the
overhead is the same for these -- one syscall -- we should batch reads
of these into the same 4k buffer. While we're at it, we can simplify a
lot of the constant dispersal.

This also adds a new test case to make sure the buffering works as
desired.

Change-Id: I7297d4aa795c00712e6484b841cef8650c2be4ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/370894
Reviewed-by: Filippo Valsorda <valsorda@google.com>
Run-TryBot: Jason Donenfeld <Jason@zx2c4.com>
Auto-Submit: Jason Donenfeld <Jason@zx2c4.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

0 files changed, 0 insertions, 0 deletions