Add comparison to "SRFast"

[awf]

Illustrate how bias-corrected stochastic rounding differs from some common implementations.

New implementations:

StochasticFastest renames the old Stochastic, implements a stochastic rounding scheme
that is biased (the rounded value is on average farther from zero than the true value).

StochasticFast implements a stochastic rounding scheme that is unbiased in
infinite precision, but biased when the quantity to be rounded is computed to
a finite precision.

Stochastic implements a stochastic rounding scheme that is unbiased with finite precision
inputs and finite SRBits.

With a lot of SRbits (say 8 or more), these biases are negligible, and there
may be some efficiency advantage in using StochasticFast or StochasticFastest.

Also produces this visualisation which illustrates how the rounding threshold is computed as a function of the input value and random bits in each case: