Generate crate interfaces and enable to import them

This adds several new features:

• we can register shims for ADTs in shim files
• RefinedRust generates a interface.rrlib file for each verified crate that specifies the public interface of this crate and the specifications it provides
• We can load these libraries with the new rr::include crate/mod-level attribute
• The frontend will search in a user-configured search path for the libraries imported via rr::include and make their specifications available to the verifier. The search path is configured via the lib_load_paths config option.