Add support for mutually recursive trait definitions

[sonmarcho]

Mutually recursive traits happen in practice, for example here (mutual recursion between PrimeField and Field):

pub trait Trait1 {
    type T: Trait2;
}

pub trait Trait2: Trait1 {}

It would be good to handle those, or at least print a nice error message. For now, Charon panics with:

thread 'rustc' panicked at 'Invalid trait decl group:
tests4::Trait1
tests4::Trait2', src/reorder_decls.rs:116:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: could not compile `tests4` (bin "tests4")