Modernize deterministic mixer code

• replace uint32_t state and free functions with a small struct
• define a single overload set for mixing
• be paranoid about implicit conversions of the 'extra' state
• use the above mentioned struct everywhere, and remove copies of the code
• add a unit test (which contains a copy of the original code to verify the results are the same)
• replace use of boost::integer_traits with std::numeric_limits