Document how to implement CLike for using EnumSet

[SimonSapin]

According to the tests, this code needed to use collections::enum_set::EnumSet:

    #[repr(uint)]
    enum Foo {
        A, B, C
    }

    impl CLike for Foo {
        fn to_uint(&self) -> uint {
            *self as uint
        }

        fn from_uint(v: uint) -> Foo {
            unsafe { cast::transmute(v) }
        }
    }

If it is indeed needed, that should be documented.

Also, it would be nice to not have to duplicate this impl for every enum type that one wants to use with EnumSet.

[Gankra]

EnumSet is not in std lib anymore.