* rand() use XOR to break up unwanted pair correlation
This form of rand() cannot return the same value twice in a row.
Two additonal EOR instructions produce a more even distribution of successive pairs.
see comments on #951
* rand.s document purpose of XOR
* suggested srand() optimization: zero fill unnecessary
* test to validate implementation of rand()
* srand() improving behaviour and adding startup test
* srand() with a tail call to rand() for better initial shuffle
* srand() can fall through to rand() instead of tail call
