Implement a Trace trait for types that can be traced.
Implement a Trace trait for types that can be traced.

We expose several JSAPI functions that are used for tracing. By abstracting these functions behind
a trait we can overload them on the type to be traced.

servo/rust-mozjs,jdm/rust-mozjs,nnethercote/rust-mozjs,nnethercote/rust-mozjs,jdm/rust-mozjs,nnethercote/rust-mozjs,jdm/rust-mozjs,nnethercote/rust-mozjs,servo/rust-mozjs,servo/rust-mozjs,nnethercote/rust-mozjs,jdm/rust-mozjs,jdm/rust-mozjs,servo/rust-mozjs