Add a bitflags! macro

The `bitflags!` macro generates a `struct` that holds a set of C-style bitmask flags. It is useful for creating typesafe wrappers for C APIs.

For example:

~~~rust
bitflags!(Flags: u32 {
    FlagA       = 0x00000001,
    FlagB       = 0x00000010,
    FlagC       = 0x00000100,
    FlagABC     = FlagA.bits
                | FlagB.bits
                | FlagC.bits
})

let e1 = FlagA | FlagC;
let e2 = FlagB | FlagC;
assert!((e1 | e2) == FlagABC);   // union
assert!((e1 & e2) == FlagC);     // intersection
assert!((e1 - e2) == FlagA);     // subtraction
~~~