Error parsing `const impl`

`const impl`s 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:

```rust
fn main() {
let r: Result = syn::parse_str(r###"const impl Foo {}"###);

println!("{:?}", r);
}
```

will output

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