std::rand: wrappers for floats from [0,1] and (0,1).

Provide `Closed01` and `Open01` that generate directly from the
closed/open intervals from 0 to 1, in contrast to the plain impls for
f32 and f64 which generate the half-open [0,1).

Fixes #7755.
auto merge of #10542 : huonw/rust/open01, r=alexcrichton

Provide `Closed01` and `Open01` that generate directly from the
closed/open intervals from 0 to 1, in contrast to the plain impls for
f32 and f64 which generate the half-open [0,1).

Fixes #7755.