Long-lived projections

Currently, a projection is required to be consumed in the same expression as the subscript we called was. This is fine, and it's similar to what Swift is doing. However, we can extend this notion like Hylo to long-lived projections:

```ferlium
let mut xs = [1, 2, 3];
let x: &mut int = xs[0]; // project out a part from xs - xs becomes temporarily inactive

x += 2; // last use

print(xs); // now xs is available again
// [3, 2, 3]
```

I suspect this requires significant additional complexity in the compiler, but it might be worth it depending on the appetite of the language. What do you think?

The issue provides a language-design example for long-lived projections but names no files, tests, or compiler entry points. Research would need to identify the projection and borrow-checking implementation first; the desired result is that a projection remains usable until its last use and then makes the original value available again.

Written by the indexing model from the issue text.