Sereal::Merger: add sub append_all() method
Sereal::Merger: split main loop into recursive functions calls

A set of srl_merge_* function has same performance as one big main loop,
but have a benefit of easier to maintain code
Sereal::Merger: make caler fill fields in STRTABLE_entry

Profilling shows that BODY_POS_OFS() macro is quite expensive when
it called millions of times. So, by letting the caller be responsible
of filling in tag_offset field I safe on BODY_POS_OFS() call when there
is already duplicate in strtable.

One more thing is that this step is an easy one because the caller is
already responsibe for setting str_offset field.
Sereal::Merger: remove parser_stack
srl_buffer: BUF_DONE macro
Sereal::Merger: small sanity fixes + comments
Sereal::Merge: fix cnt_of_merged_elements
remove code
Sereal::Merger: start writting tests (001_load.t)
Sereal::Merger: more tests (004_testset.t)