add support for operator overloading via ops traits

[sbillig]

Notes:

• Unary plus is currently hardcoded as a no-op. I think we should remove parser support for it. I dunno what it would do. (rust doesn't support it, fwiw)
• The index op on arrays can't be implemented in fe yet; impl<T, const N: usize> Index<usize> for [T; N] fails on the use of N in [T; N] (we only int literals in that position right now) (issue add support for using const ty param as array size #1120)
• Unlike rust, we currently have a BitNot trait in addition to Not. We could merge them.

eg

use core::ops::Add
impl Add<i32> for Point {
  fn add(self, _ n: i32) -> Point {
    Point { x: self.x + n, y: self.y + n }
  }
}

fn f(p: Point) -> Point {
  p + 100
}