Sereal::Merger: 003_strtable.t + small 003_strtable.t modifications
Sereal::Merger: simplify dedupe logic

In Sereal::Encoder deduplications of keys comes for free.
It's not true for Sereal::Merger, the cost of deduping keys is equal to
cost of deduping any other strings.
Sereal::Merger: add support of OBJECT and OBJECT_FREEZE tags
Sereal::Merger: add support of REGEXP tag
Sereal::Merger: add support of SRL_HDR_OBJECTV and SRL_HDR_OBJECTV_FREEZE tags