Sereal::Merger: an attempt to use common srl_error.h

When I split srl_error.h out of decoder code I though that I can easily
reuse that code. I was wrong. SRL_ERROR* macros have a strong ties to
srl_decoder.c. So, I had to copy code in a new header and fix SRL_ERROR*
macros to be more generic but didn't invest much time in it though.

Need a revisit!
Sereal::Merger: add support of snappy decompression