WIP: const effect

[tiif]

Opened a draft so it is easier to fetch and push to this branch.

[nikomatsakis]

Ignore this, I'm making a test :)

[tiif]

Revisited this, I think there are still more stuff needed to properly model this. It might be ok to just rebase and land this, but the syntax we use here is definitely not the friendliest to the reader >:)

This has happened a while ago, so to recap, we defined the subset relationship between const, runtime and associated effect.

How the test currently looks like for

 const fn foo<T>()
 where 
   T: Default,
 {
     <T as Default>::default();
 }

is

fn foo() -> () random_keyword do const {
     (AssociatedEffect(Default()))
}

Improvements

1. Now that we already have some basic support for minirust bodies (yay!), technically we can just support fully qualified path in function body, similar to what we did for ValueExpression::Fn, but the type checking for fully qualifed path might be more involved.

2. It'd be nice to match the syntax that is planned to be used, and write something similar to

const fn default<T: [const] Default>() -> T {
    T::default()
}

but we might need to do a small refactoring to make this happen.
3. Fix the parser bug that requires us to add random_keyword to make the grammar work :)