Fix Issue #5240: faster std.random.uniform for [0.0, 1.0) range

Following typical design in other languages, the function is named
uniform01.  It can be templated on floating-point type, defaulting
to double if none is specified.

The implementation is pretty much derived from its counterpart in
Boost.Random, in particular in its taking account of the different
requirements for integral versus floating-point RNG values, and in
checking that the generated variate is less than 1 before returning.
This last check is necessary simply because we can't guarantee, in
the case of a floating-point based RNG, that we will not get a
result exactly equal to 1.

Fixes http://d.puremagic.com/issues/show_bug.cgi?id=5240