This adds support for computing the quantiles of a mutator utilization
distribution.

Change-Id: Ia8b3ed14bf415c234e2f567360fd1b361d28bd40
Reviewed-on: https://go-review.googlesource.com/c/60799
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>