Write a lint that ensures that no_jsmanaged_fields is used correctly

[jdm]

We should be able to check all of the fields of the target of the no_jsmanaged_fields macro and ensure that none of them are marked jsmanaged. This will prevent misuse of the macro.

[leavengood]

To add some more detail, it seems one would want to look for the jstraceable attribute, as defined in https://github.com/servo/servo/blob/master/components/plugins/jstraceable.rs

An existing lint which checks for attributes is https://github.com/servo/servo/blob/master/components/plugins/lints/privatize.rs

This new lint would need to find calls to the no_jsmanaged_fields macro, and then look at the fields of any structs passed to it to see if the jstraceable attribute is defined on any of the fields.

I think (Servo newbie)

[jdm]

Whoops, I meant must_root when I said "none of them are marked jsmanaged". jstraceable isn't strong enough for our purposes here.