Heapster: existential function output perms

[eddywestbrook]

This PR adds existential ghost variables to the output of a function permission, so that now the general form of a function permission looks like this:

(x1:tp1, ..., xn:tpn). arg0:p0, ..., argm:pm, x1:p1', ..., xn:pn
-o (z1:tp1, ..., zk:tpk). arg0:p0'', ..., x1:p1''', ..., ret:p, z1:p1'''', ...

This change was needed to support Rust functions that return structs or other data structures that contain pointers with lifetimes as subterms. For example, a function that returns a slice type like &'a str actually returns a struct with a length and a pointer in lifetime 'a, and the pointer needs a variable, which we can now list in the output ghost variables, in order to reference it in the returned lifetime ownership permission.

As an added bonus, this PR also fixes lownedPermVarAndPerm, lownedPermsVars, and lownedPermsToDistPerms to accommodate for offsets on variables, thereby removing errors when ending lifetimes with offset variables in the lowned permissions.

[m-yac]

LGTM, though I wasn't able to review the changes in RustTypes.hs beyond just general Haskell style since I don't know what goes on in that file. I did have a couple questions so I'm not hitting "Approve" yet.