syntax: correct the Rand::rand call to select enum variants in #[deriving(Rand)].

Previously, this was not a global call, and so when `#[deriving(Rand)]`
was in any module other than the top-level one, it failed (unless there
was a `use std;` in scope).

Also, fix a minor inconsistency between uints and u32s for this piece
of code.
auto merge of #7101 : huonw/rust/rand-call, r=erickt

Previously, this was not a global call, and so when `#[deriving(Rand)]`
was in any module other than the top-level one, it failed (unless there
was a `use std;` in scope).

Also, fix a minor inconsistency between uints and u32s for this piece
of code.