Add JS_NumberValue to glue.
Expose JIT property op helpers, and make jsglue use the C++ Spidermonkey API in the process.
Expose more #define values as constants.
Expose some friend APIs and random flags.
Expose proxy handlers.
Expose some more proxy helpers, and don't run methods that can run arbitrary function pointers with #[rust_stack].
Make handle objects work right, expose some jsid stuff.