WIP: Stable rust support

[whentze]

This is my attempt at making this crate (at least with default features) work on stable rust. Mostly just putting this up to see what you think, so it's a draft PR for now.

It adds a proc macro dependency and makes the code uglier, so I understand if you think this is not worth the trouble.

[nbdd0121]

The c_unwind shouldn't be removed until the final c_unwind stabilisation completes.

One issue with the naked function macro is that it will export additional symbols, where built-in naked function will not.