Reduce register spilling

[ogxd]

Context

The gxhash rust implementation is cut into several functions to ease maintenance, however most of them are inlined in the final byte code. This makes it more difficult to spot registry spilling, but it seems some optimizations could be made in this regard.
A hint that we are close to spilling is that the slightest change currently has a very negative impact on performance.

Todo

Take a look at the generated X86 asm code (cargo asm), and look for potential optimizations in usage of registries.