Use FooValue() functions.
Use the JSVal member functions to replace JSVAL_IS_* and JSVAL_TO_*.
Update rust-mozjs to include fixes for GC crashes.
Split JSValConvertible into two traits.

Later, we'll want to implement variations of the conversion *from* JS, but
not the conversion *to* JS.
Pass a JSContext to ToJSValConvertible::to_jsval.
Replace str_to_jsval and domstring_to_jsval with a JSValConvertible implementation.