Support namespace attributes in the webidl frontend

[fitzgen]

We don't yet support attributes within namespaces:

namespace Whatever {
    readonly attribute bool blah;
}

Note that attributes within namespaces are always readonly.

Would need to add support here: https://github.com/rustwasm/wasm-bindgen/blob/master/crates/webidl/src/lib.rs#L926

[fitzgen]

This might be a blocker for #613 -- I need to check how often this happens in practice and how important the instances are.

[fitzgen]

This might be a blocker for #613 -- I need to check how often this happens in practice and how important the instances are.

There aren't any in our enabled webidls. Don't think this is a blocker!

[alexcrichton]

Since this doesn't show up in any of our webidl and it doesn't look like namespaces are really used much any more I'm gonna go ahead and close this