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

Cargo.toml

@@ -8,6 +8,9 @@ edition = "2018"
 generate = { path = "crates/generate" }
 memory = { path = "crates/memory" }
 
+[dev-dependencies]
+proptest = "0.9"
+
 [workspace]
 members = [
   "crates/generate",