Implement support for capital Self in traits and impl blocks

[cburgdorf]

What is wrong?

We currently do not support Self in traits and impl blocks (See Rust docs).
Implementing support for Self would give us an easy way to unlock certain functionality even before traits are allowed to be fully generic (which is currently still blocked until we get rid of Location in the analyzer).

E.g. here is how min() / max() functions could be supported.

trait Min {
  fn min() -> Self
}

trait Max {
  fn max() -> Self
}

impl Min for i8 {

  fn min() -> i8 {
    return -128
  }
}

impl Max for i8 {

  fn max() -> i8 {
    return 127
  }
}

// Then somewhere else
let x: i8 = i8::min()
let y: i8 = i8::max()

How can it be fixed

• Add support for Self in parser
• Add support for Self in analyzer
• Lower Self to actual type
• Add tests

[cburgdorf]

This landed