optimize small Getters

After some profiling, i came to the conclusion that current running logic is too complex, in order to support `bytesRead`, we have two `Decoder` type, the overhead of `runGetIncremental` is noticeable when the getter is small(around 100ns compare to cereal on my machine).

This gives binary a disadvantage in various benchmarks, since lots of benchmark just focus on a single combinator, the overhead of running it render this kind of benchmarks meaningless.

There're cases we do need running small getters, but i haven't really need `bytesRead` yet. So let's find a way to improve `bytesRead`, or remove it ?