Add support for rawptrs

[ThomasMayerl]

This PR adds support for rawptrs by adding Delegation Edges that connect rawptrs to their derived from places when this fact is known. Applying an action to a rawptr deref degelates that PCG action to the derived from place. This PR also adds RawPtrDeref edges that work similar to normal Deref edges - just that we do not have a lifetime projection when we do not know where the rawptr was derived from.
Note: The borrow checker does not check if rawptrs can be accessed. Nor does the PCG. However, this PR should show how capabilities change for references/places once rawptrs are involved.

[zgrannan]

@ThomasMayerl The code changes themselves look reasonable, but before merging please fix the failing test. It looks like the error is: Place (**node).next projects a shared ref, but has capability E. I haven't looked at the test case, but I suspect that node is a shared reference, *node is a *mut pointer, and therefore **node has exclusive capability. If that's the case, then it may simply be a matter of refining the validity check.

[zgrannan]

@ThomasMayerl This mostly looks good; the capability fix is a bit of a workaround for some other underlying issue: namely, the capability for the blocked places should already be correctly set via AbsExpander.expand_to_places. However, since the workaround doesn't seem to be breaking anything, I'll leave it in for now. At some point, I'm going to revisit the loop implementation to address #162 so hopefully the underlying issue can be fixed at that time.