Support reexport for procedural macros

[taiki-e]

Create a hidden module that reexports all items in the crate and import it at the beginning of the wrapper macro. This allows supporting not only rename but also reexport.

#[doc(hidden)]
pub mod __reexport {
    #[doc(hidden)]
    pub use crate::*;
}

#[doc(hidden)]
#[proc_macro_hack(support_nested)]
pub use futures_macro::join_internal;

#[macro_export]
macro_rules! join {
    ($($tokens:tt)*) => {{
        // The code generated by proc-macro uses a path starting with `__futures_crate`.
        // Also, `use $crate;` in macro is not allowed (https://github.com/rust-lang/rust/issues/37390),
        // so create hidden module `__reexport`.
        use $crate::__reexport as __futures_crate;
        $crate::join_internal! {
            $( $tokens )*
        }
    }}
}

Note that proc-macro is renamed to *_internal.
IIRC, it is not expected to use futures-macro directly, so this is probably fine...
(Maybe using = requirements would help to avoid some problems like #2061.)

Closes #2099

[cramertj]

Nice, thanks for fixing this!