fast portable 256-bit SIMD pseudo random number generators.

!!! TIP This is an alpha version

Implements two generators:

Required compiler features:

• C11 for standard atomic operations
• GCC vector_size attribute

Required SIMD hardware operations (for performance):

• 64-bit addition
• 64-bit shift by constant (same per lane)
• 32-bit product (low 32-bit result)

Example variants can up the ante on hardware operations.

For speed it requires one more logical operation (an xorshift) vs. SplitMix64 (SIMD instead of scalar ops)