random -- effect that provides random numbers
for a random instance installed in the current environment, update the
environment to the next random value. The the contents of the current
instance are left unchanged, 'random.env' will provide the new instance.
return next random number of type f32 in the range [0..1),
result will never be equal to f32 1
return next random number of type f64 in the range [0..1),
result will never be equal to f64 1