Macro register_plugin 

macro_rules! register_plugin {
    (manifest = $manifest:expr $(, $($tail:tt)*)?) => { ... };
}

Generate hm_manifest + capability exports for a plugin.

Example

register_plugin!(
    manifest = ...,
    executor = MyExec,
    hook     = MyHook,
);

// Order-independent: this is equivalent.
register_plugin!(
    manifest = ...,
    hook     = MyHook,
    executor = MyExec,
);