callOnPathSteps was using linear scan search in KeyedObjectContainer.
This commit fixes it for using binary search, dramatically improving
performance in workloads that do lots of lookups.
This commit fixes a couple of bugs preventing SortedVector and
MutableSet from being serialized.
Also, it introduces minimal testing for serialization.
These data structures are substitutes for a `std::map<Key, Value>` where
`Key` is embedded in `Value`. Their main goal is to be serializable in a
YAML sequence while preserving the order enforced by the key.
`MutableSet` is implemented using a map.
`SortedVector` is implemented using a sorted vector.
Ivan Krysak