Some random number generates output floating point numbers directly, so
by providing these methods all the functionality in librand is available
with high-performance for these things.
An example of such an is dSFMT (Double precision SIMD-oriented Fast
Mersenne Twister).
The choice to use the open interval [0, 1) has backing elsewhere,
e.g. GSL (GNU Scientific Library) uses this range, and dSFMT supports
generating this natively (I believe the most natural range for that
library is [1, 2), but that is not totally sensible from a user
perspective, and would trip people up).
Fixes https://github.com/rust-lang/rfcs/issues/425.
e49be7aae3