Kruglov/optimize go decoder

With this set of commits the performance of Go decode was improved by factor of ~3.5x

result of old code:
sereal-go $ go run dec.go -file data_go.srl
read file: data_go.srl
deserialize data
14022
Deserialization took 5.542141726s to run.

result of new code:
sereal-go $ go run dec.go -file data_go.srl
read file: data_go.srl
deserialize data
14022
Deserialization took 1.592267889s to run.

I've benchmarked the code with ad-hoc script:
https://github.com/ikruglov/junk/blob/master/sereal-go/dec.go

Please refer to commit's comments for more details about what was done.