libcore: rand refactor & remove @'s from {rand,run}.rs

From a cursory `git grep` this removes the last part of `core` that requires on `@` (other than `io` and the task local data section).

It renames `RandRes` to ~~StdRng~~ `IsaacRng` and `XorShiftState` to `XorShiftRng` as well as moving their constructors to static methods. To go with this, it adds `rng()` which is designed to be used when the programmer just wants a random number generator, without caring about which exact algorithm is being used.

It also removes all the `gen_int`, `gen_uint`, `gen_char` (etc) methods on `RngUtil` (by moving the defintions to the actual `Rand` instances). The replacement is using `RngUtil::gen`, either type-inferred or with an annotation (`rng.gen::<uint>()`).

I tried to have the `Rng` and `RngUtil` traits exported by `core::prelude` (since `core::rand` (except for `random()`) is useless without them), but this caused [an explosion of (seemingly unrelated) `error: unresolved import`'s](https://gist.github.com/5451839).