Fix the incorrect free lifetime on types::Callback

[jugglerchris]

As copied from #172 (comment)

rlua contains within its guts a dirty lie, and this lie permeates the entire crate and makes proving the soundness of rlua very very hard. The signature of this type alias SHOULD be:

pub(crate) type Callback<'a> =
    Box<for<'lua> dyn Fn(Context<'lua>, MultiValue<'lua>) -> Result<MultiValue<'lua>> + 'a>;

Making this change would immediately remove much of the very delicate and honestly sort of sketchy logic in the callback creation process and the entirety of the "scope" system and make it more obviously sound. However, since I screwed this up when I initially created rlua and wrote the type signature wrong, I "accidentally" found a way to make it so that callback creation did not require macros. This has prevented me from fixing this because doing so almost certainly makes the API even less convenient by requiring macro wrappers around callbacks. There might be a way to fix this but I've tried very hard and haven't found any solution, it almost certainly requires GATs to fix properly. Edit: This is the biggest blocker in my mind to a 1.0 level API, but maybe being able to generally yield in callbacks also deserves to be a 1.0 blocker.