Sereal::Merger: add support of different top level elements
Sereal::Merger: more tests (010_desperate.t)
Sereal::Merger: add support of dedupe_strings option
ptable: make PTABLE_store return pointer to table entry

There is a need to update value of already inserted record in
Sereal::Merger.
Sereal::Merger: need to update tracked_offset_table before going to recursion
Sereal::Merger: add support of ALIAS tag
Sereal::Merger: make a shortcut for PAD tag