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 ?

Start by examining the two Decoder types and the runGetIncremental path described in the issue, then reproduce the reported small-getter overhead with the existing benchmarks. Compare the bytesRead requirement with the cases that do not need it and determine whether it can be improved or removed. Done means reducing the small-getter overhead without losing required bytesRead behavior.

Written by the indexing model from the issue text.