Macro cleanups

[nnethercote]

These make the code easier to read, and the docs much nicer. Details in individual commits.

[nnethercote]

Here's what the old docs look like:

macro_rules! cfg_if {
    (
        $(
            if #[cfg( $i_meta:meta )] { $( $i_tokens:tt )* }
        ) else+
        else { $( $e_tokens:tt )* }
    ) => { ... };
    (
        if #[cfg( $i_meta:meta )] { $( $i_tokens:tt )* }
        $(
            else if #[cfg( $e_meta:meta )] { $( $e_tokens:tt )* }
        )*
    ) => { ... };
    (@__items ( $( $_:meta , )* ) ; ) => { ... };
    (
        @__items ( $( $no:meta , )* ) ;
        (( $( $yes:meta )? ) ( $( $tokens:tt )* )) ,
        $( $rest:tt , )*
    ) => { ... };
    (@__identity $( $tokens:tt )* ) => { ... };
}

and the new docs:

macro_rules! cfg_if {
    (
        if #[cfg( $i_meta:meta )] { $( $i_tokens:tt )* }
        $(
            else if #[cfg( $ei_meta:meta )] { $( $ei_tokens:tt )* }
        )*
        $(
            else { $( $e_tokens:tt )* }
        )?
    ) => { ... };
}

Much better!