This reverts commit a482734.

This reverts commit e8a9292.

    error[E0502]: cannot borrow `generics.params` as mutable because it is also borrowed as immutable
      --> tests/test_iterators.rs:81:25
       |
    77 |       let _ = generics
       |               --------
       |               |
       |  _____________immutable borrow occurs here
       | |
    78 | |         .lifetimes()
       | |____________________- a temporary with access to the immutable borrow is created here ...
    ...
    81 |           .unwrap_or_else(|| {
       |                           ^^ mutable borrow occurs here
    82 |               let lifetime: Lifetime = parse_quote!('a);
    83 |               generics.params.insert(
       |               --------------- second borrow occurs due to use of `generics.params` in closure
    ...
    88 |           });
       |             - ... and the immutable borrow might be used here, when that temporary is dropped and runs the destructor for type `impl Iterator<Item = &LifetimeParam>`