Const generics not contained in a block

[dtolnay]

The implementation from #261 parses RectangularArray<i32, {2}, {4}> but RFC 2000 allows no block for a generic argument that is a single literal.

extern crate syn;

fn main() {
    let ty = "RectangularArray<i32, 2, 4>";
    println!("{:#?}", syn::parse_str::<syn::Type>(ty).unwrap());
}