Parse trait bounds containing associated constant constraint

[dtolnay]

As seen in src/test/ui/associated-consts/assoc-const.rs:

fn foo<F: Foo<N = 3usize>>() {}

Syn can't currently parse this.

error: expected one of: `for`, parentheses, `fn`, `unsafe`, `extern`, identifier, `::`, `<`, square brackets, `*`, `&`, `!`, `impl`, `_`, lifetime
  --> src/test/ui/associated-consts/assoc-const.rs:18:17
   |
18 | fn foo<F: Foo<N=3usize>>() {}
   |                 ^^^^^^

[dtolnay]

• Feature request: Equality constraints on associated constants rust-lang/rust#70256
• Tracking issue: Tracking issue for associated const equality rust-lang/rust#92827