Refactor frontend/middleware types (#194)

* unify fe/be NativeOp and NativePred

* remove Origin in favour of PodId

* Combine string and hash in Key

* use middleware::AnchoredKey in frontend

* merge frontend/middleware types

* refactor custom predicates

* clean up a bit

* fix middleware custom tests

* clean up

* clean up 2

* add acronyms in typos list

src/lib.rs

@@ -4,7 +4,6 @@ pub mod backends;
 pub mod constants;
 pub mod frontend;
 pub mod middleware;
-mod util;
 
 #[cfg(test)]
 pub mod examples;