rhg: in path_encode add a DestArr type This is an implementation of Sink trait that writes into a fixed-size buffer on the stack, so identical to what was done before, but it makes the code of [hash_encode] easier to understand by dropping all these slice manipulations.

rhg: reduce verbosity in path_encode by using a trait for writing Hopefully this makes the code easier to read and understand and shorter overall. It also lets us later tweak the type we use as a [Sink], without having to change the encoding functions, including using two different types for size measurement and for the actual serialization.