This commit replaces the ue2::unordered_{set,map} types with their STL
versions, with some new hashing utilities in util/hash.h. The new types
ue2_unordered_set<T> and ue2_unordered_map<Key, T> default to using the
ue2_hasher.
The header util/ue2_containers.h has been removed, and the flat_set/map
containers moved to util/flat_containers.h.
This commit replaces the build-time representation of the Rose
interpreter programs, from a class containing a discriminated union of
the bytecode structures to a class hierarchy of build-time prototypes.
This makes it easier to reason about and manipulate Rose programs during
compilation.
