Parse lifetime args inside of turbofish

[dtolnay]

This is apparently legal Rust syntax. It takes some effort to come up with code with lifetimes inside turbofish which rustc will accept without hitting rust-lang/rust#42868, but here is one such example:

struct Struct;

impl Struct {
    fn f<'a: 'a>(&'a self) {}
}

fn main() {
    Struct.f::<'static>();
}

Syn currently doesn't parse this syntax, it assumes the turbofish contains only types and constant expressions, and treats the lifetime as if it were a bound of a 2015-edition syntax trait object type.

error: at least one trait is required for an object type
 --> dev/main.rs:8:16
  |
8 |     Struct.f::<'static>();
  |                ^^^^^^^