cmd/trace: add minimum mutator utilization (MMU) plot

This adds an endpoint to the trace tool that plots the minimum mutator
utilization curve using information on mark assists and GC pauses from
the trace.

This commit implements a fairly straightforward O(nm) algorithm for
computing the MMU (and tests against an even more direct but slower
algorithm). Future commits will extend and optimize this algorithm.

This should be useful for debugging and understanding mutator
utilization issues like #14951, #14812, #18155. #18534, #21107,
particularly once follow-up CLs add trace cross-referencing.

Change-Id: Ic2866869e7da1e6c56ba3e809abbcb2eb9c4923a
Reviewed-on: https://go-review.googlesource.com/c/60790


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>