cmd/compile: add explicit 'where' to EscStep data for explanations
Sometimes neither the src nor the dst of an escape edge
contains the line number appropriate to the edge, so add
a field so that can be set correctly.
Also updated some of the explanations to be less jargon-y
and perhaps more informative, and folded bug example into
test.
Cleaned up some of the function/method names in esc.go
and did a quick sanity check that each "bundling" function
was actually called often enough to justify its existence.