Affects Version/s: None
Fix Version/s: None
We have multiple mapping types defined using the TYPED_MAP_DEFINE and TYPED_MAP_DECLARE macros, in particular the StringMap is used very often. There are also places  where we need a mapping from char* to Sequence, yet we don't have a type for this mapping that would be shared among sources.
 see libpromises/eval_context.c and report packing
1. shared StringSequenceMap type defined and used in places that need it
2. extra functions for this particular mapping like StringSequenceMapAppend(map, key, item) which appends to the sequence map[key] and creates it if necessary and possibly some other functions that are combinations of the map operations and sequence operations
3. unit tests for the new type