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.
Previously, we disallowed the merging of all Rose prefixes in block mode
where the literal sets are not identical.
This change allows merging if the prefix graphs to be merged are very
small, as a small performance improvement for cases with lots of tiny
prefixes.
This check is deliberately conservative: graphs must have some common
vertices, and the result of the merge must not give up any
accelerability.
