- paths can now take region parameters, replacing the dirty hack
I was doing before of abusing vstores. vstores are now a bit
of a hack though.
- fix various small bugs:
- we never checked that iface types were compatible when casting
to an iface with `as`
- we allowed nonsense like int<int>
- and more! (actually that may be it)
- we now distinguish bound/free parameters (see region-param
test case for why this is necessary)
- we also track bounds on region variables
- also, restructure fold_ty() to have multiple variants without
duplication instead of one overloaded folder. This also allows
for using block functions.
