Refactor tests to use proptest (#6)

* 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

crates/memory/src/memory.rs

@@ -26,7 +26,7 @@ impl<'a> GuestMemory<'a> {
     fn contains(&self, r: Region) -> bool {
         r.start < self.len
             && r.len < self.len // make sure next clause doesnt underflow
-            && r.start < (self.len - r.len)
+            && r.start <= (self.len - r.len)
     }
 
     pub fn ptr<T: GuestType>(&'a self, at: u32) -> Result<GuestPtr<'a, T>, GuestError> {