Remove _non_exhaustive fields to use #[non_exhaustive] macro

[a1phyr]

Remove _non_exhaustive fields and replace them with the new #[non_exhaustive] macro

Pros

• Disallow users to use this field anyway, which limit future breaking changes
• Cleaner

Cons

• Rust version support is reduced to 1.40+ (I didn't see any notice about that so it may be fine)
• Breaking change
• Disallow let handle = Handle { /* fields */ ..Handle::empty() }; syntax

Maybe provide a Handle::new(...) function for each type ?

[Lokathor]

I like the idea of this, particularly because by now we're about on to 1.50, but I'll need to sit and think about it to be sure.

[Lokathor]

Alright, I'm interested in having this in for sure.

@a1phyr there are some merge conflicts it seems. Can you update your PR to fix those? Or if you're not interested any more I can sort it out later.

[a1phyr]

Rebased, added Redox, and fixed doc-tests