uniform_01 generator

There's this comment in `uniform_01.hpp`:

> Note: The current implementation is buggy, because it may not fill
> all of the mantissa with random bits. I'm unsure how to fill a
> (to-be-invented) @c boost::bigfloat class with random bits efficiently.

We do have an efficient algorithm ([see section 4 of this preprint](https://arxiv.org/pdf/1810.04744.pdf)) and a [C++ implementation](https://github.com/DiscreteLogarithm/canonical-random-float) capable of generating all representable floating-point numbers in [0,1). It fills the mantissa with a random integer and generates a geometric random number for the exponent. In the double precision case, it is even faster than simply multiplying a 64-bit integer by 2^-64 and in the single precision case it's only 25% slower than multiplying a 32-bit integer by 2^-32 (when used with the MT19937 URBG).

If you are interested we can help to integrate it into boost.