Sereal::Merger: an attempt to use faster hash table

Initially I used builtit Perl's hash tables for string deduplication.
Unfortunately, they have a lot of overheades which were clearly visible
in profile.

As an attempt to improve this matter I've created STRTABLE. Its
implementation is mostly derived from ptable.h and hv.c/sv.c
with set of optimization such as zero string copying and others.