Part of Pin Ergonomics.

This forbids places to be moved or mutably borrowed ever since they are pinned until they are reassigned (even after the pinned borrows themselves expire).

#![feature(pin_ergnonmics)]
#![allow(incomplete_features)]

#[pin_v2]
struct Foo;

fn foo(mut x: Foo) {
    {
        let _x = &pin mut x; // x is pinned
    } // now the pinned reference expires
    let _x = &mut x; //~ ERROR cannot borrow `x` as mutable because it is pinned
    let _x = x; //~ ERROR cannot move out of `x` because it is pinned
    
    x = Foo; // x is reassigned
    let _x = &mut x; // ok because x is reassigned and not pinned yet
}

Tasks:

• Add Pins dataflow analysis
• Improve the diagnostics of pinning errors
• Test pin pattern matching
• Forbid &pin borrows for ADTs without #[pin_v2] (for soundness concerns)

Unresolved questions

• Should we allow &pin borrows to be applied to generic ADTs without #[pin_v2]? (related discussions: Require #[pin_v2] for direct &pin borrows of ADTs frank-king/rust#2 (comment))
• Should &pin borrowed places of Unpin types be prevented from being moved, or just don't treat Unpin types specially and emit move-after-pin as usual?