Transform associated types into type parameters

[Nadrieril]

This PR introduces a --remove-associated-types <name pattern> option that replaces the associated types of the selected traits and converts them into type parameters.

Fixes #127.

This has 4 know limitations today:

• Bug: type aliases don't work with --remove-associated-types #531;
• Bug: incorrect generics when implemented method has more specific signature #513;
• Doesn't yet work with dyn Trait (Add support for dyn Trait #123);
• We erase some lifetimes in quantified clauses like T: for<'a> Trait<'a, Type = &'a ()> (Bug: --remove-associated-types doesn't track quantified lifetimes #534).

This also has three fundamental limitations that we can't work around:

• We can't handle GATs, as this would require passing type-level functions as type parameters;
• We can't handle for<'a> clauses with unconstrained associated types, for the same reason;
• We can't add parameters to self-recursive traits, as this would require an infinite list of parameters.

[sonmarcho]

This is really awesome to see this finally landing, thanks a lot for this! And specifying which traits this should be applied on by using a pattern is a good design choice.

I have one question: by <'a> clauses with unconstrained associated types you mean something like this?

trait Foo<'a> {
  type Bar;
  ...
}

fn f<T>(...) where for<'a> T : Foo<'a>

[Nadrieril]

I have one question: by "<'a> clauses with unconstrained associated types" you mean something like this?

Yep exactly, because we'd have to translate that to something like

fn f<T, Bar<'_>>(...) where for<'a> T : Foo<'a, Bar<'a>>

which doesn't exist in rust.