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.
Replace the use of the internal_report structure (for reports from
engines, MPV etc) with the Rose program interpreter.
SOM processing was reworked to use a new som_operation structure that is
embedded in the appropriate instructions.
