feat: Implement parsing of traits

[jfecher]

Description

Problem*

Working towards #527

Summary*

Adds support for parsing:

• trait definitions
• trait implementations
• trait constraint expressions on functions

Since traits as a whole are unimplemented, I added a parsing error to alert users that traits are still unimplemented. This will allow use to better split traits into smaller PRs like this one rather than implementing them all at once.

Documentation

• This PR requires documentation updates when merged.

  • I will submit a noir-lang/docs PR.
  • I will request for and support Dev Rel's help in documenting this PR.

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[jfecher]

Here's an example that now parses:

trait Foo<A, B> {
    type Element;

    fn iter<C>(&mut self, stop: bool) -> Field 
        where C: Foo<C, Element>;
}

impl<A> Foo<A, Field> for u32 where A: Bar {
    type Element = u8;

    fn iter<C>(&mut self, stop: bool) -> Field 
        where C: Foo<C, u8>
    {
        if stop { 0 } else { 1 }
    }
}

fn bar<T>(x: T) -> T where T: Bar {
    x.bar()
}