Change random number generator from MT19937 to xoroshiro128+
Use xoroshiro128+ as the RNG instead of MT19937. A (slow) portable version is implemented, along with a fast version for both x86 and sparc. This new generator has a much smaller state (an array of double-float values, an unsigned 32-bit integer, a slot whose value is NIL or T). This is far smaller than the 600+ word state needed for MT19937.
Tests indicate that the generator (for x86 and sparc) is a bit faster as well.
Fixes #48 (closed)