Added --subplot* to plotmpl.py

Driven primarily by a want to compare measurements of different runtime
complexities (it's difficult to fit O(n) and O(log n) on the same plot),
this adds the ability to nest subplots in the same .svg which try to align
as much as possible. This turned out to be surprisingly complicated.

As a part of this, adopted matplotlib's relatively recent
constrained_layout, which behaves much more consistently.

Also dropped --legend-left, no one should really be using that.