Feat/improve pin serial

[lanzafame]

Merge #597 first.

Benchmarks of PinSerial.ToPin() and PinSerial.DecodeCid():

BenchmarkPinSerial_ToPin-8               2000000              4322 ns/op            1680 B/op         18 allocs/op
BenchmarkPinSerial_DecodeCid-8           5000000              1456 ns/op             576 B/op          6 allocs/op

I don't expect ToPin() to be efficient but we call it quite a lot just to get the decoded Cid value out, especially in the rpc functions (14 occurances of c := pinSerial.ToPin().Cid) which will be a win for rpc calls like Status which gets called a lot.

[coveralls]

Coverage decreased (-0.03%) to 64.898% when pulling 4f194f5 on feat/improve-pin-serial into a0dc644 on master.