Walking up the tree to find the enclosing SourceFile would distort the
timing too much so, instead, we attach a Path in the binder.
At present, the path is determined retroactively by walking up the call
stack in the trace visualizer, but this is both inconvenient and
routinely inaccurate (checking an expression in one file may require
checking an expression in another file and there's no way to determine
from the trace where this transition occurred).
Andrew Casey
7e3eccedd7