Before this commit, the following code would fail to compile...
obj.foo.bar = True
... if foo is marked kernel_invariant in obj, even if bar is not
marked as such in obj.foo.
This enables constant propagation optimisations, as verified by
the included test case. This is only a first stop-gap measure, though;
we should support optimisation based on kernel invariants on a more
fine-grained level.
The core of the problem that 373578bc was attempting to solve is
that diagnostics sometimes should be chained; one way of chaining
is the loc.expanded_from feature, which handles macro-like expansion,
but another is providing context.
Before this commit, context was provided using an ad-hoc override
of a diagnostic engine, which did not work in cases where diagnostic
engine was not threaded through the call stack. This commit uses
the newly added pythonparser context feature to elegantly handle
the problem.
