* generator: take an &mut GuestMemory
rather than pass the owned GuestMemory in, just give exclusive access
to it. Makes testing easier.
* tests: start transforming tests to check abi-level generated code as well
* finish lowering of test funcs
* tests: rename variables to more sensible names
* proptesting: reliably finds that we dont allow stuff to be right against end of memory!
* memory: fix off-by-one calc in GuestMemory::contains(&self, Region)
ty proptest!
also, refactored the Region::overlaps to be the same code but easier to
read.
* generator: better location information in GuestError
* testing: proptest generates memory areas, tests everything
