Upgrade the QuickCheck/rand dependencies.

This also adds a check in the synthesis of arbitary items of unknown type,
to ensure that we don't accidentally create objects with a known type. I'm
actually not 100% sure how the old tests were passing so reliably, which
is worrying.

Cargo.toml

@@ -16,5 +16,5 @@ num-traits = { default-features = false, version = "0.2" }
 thiserror  = { default-features = false, version = "1" }
 
 [dev-dependencies]
-quickcheck = "0.9"
-rand = "0.7"
+quickcheck = "1.0.3"
+rand = "0.8.3"