Error parsing `const impl`

[orium]

const impls are unstable, but used by the standard library (e.g. https://doc.rust-lang.org/stable/src/alloc/vec/mod.rs.html#908). They are currently not parsable by syn:

fn main() {
   let r: Result<syn::File, _> = syn::parse_str(r###"const impl Foo {}"###);
   
   println!("{:?}", r);
}

will output

Err(Error("expected identifier or `_`"))

[usmanovbf]

+1 for me too. Probably, I will prepare PR for that if no one minds