Low speed/hardened code vs. higher speed/low-check code

With version 0.2.1 it is possible to speed up the ringBufU instance access by using a compilation with special definitions set (RINGBUFU_FASTACC_NO>0).
Hence older versions could be used with faster ringBufU API functions yet; but also with less savety.
Also it should be tested more thouroughly. Hence i changed it to beta; hope you could find it too.