Expand doc comments to attributes before macro expansion

[brendanzab]

This would allow doc comments to parse them as $_:meta elements in macro invocations. This is useful for example with bitflags!:

bitflags! {
    #[doc = "Some flags"]
    #[doc = ""]
    #[doc = "A new line"]
    #[deriving(Show)]
    flags Flags: u32 {
        #[doc = "Flag A"]
        static FlagA = 0x00000001,
        #[doc = "Flag B"]
        static FlagB = 0x00000010
    }
}

vs:

bitflags! {
    /// Some flags
    ///
    /// A new line
    #[deriving(Show)]
    flags Flags: u32 {
        /// Flag A
        static FlagA = 0x00000001,
        /// Flag B
        static FlagB = 0x00000010
    }
}

Where the bitflags! match rule is defined as:

($(#[$attr:meta])* flags $BitFlags:ident: $T:ty {
    $($(#[$Flag_attr:meta])* static $Flag:ident = $value:expr),+
})