boxed: Use JSNative property accessors

Property accessors of JSPropertyOp and JSStrictPropertyOp are going away
in future versions of SpiderMonkey, to be replaced by JSNative.

Unfortunately the property name was passed to the PropertyOp callbacks as
a jsid, and it is not available to JSNative callbacks. The property
accessors in Boxed do require the property name, however, so they can
look up the GIFieldInfo. In order to achieve this we follow a suggestion
from a Mozilla mailing list [1]: wrap the JSNative accessors in
JSFunction objects, which can have "reserved slots" in which to store
information with which we can retrieve the field info.

For this, we need to use the jsfriend API, but such use is isolated to
two functions.

[1] https://groups.google.com/forum/#!msg/mozilla.dev.tech.js-engine.internals/TZznbH80zJw/BmRrMsuuAwAJ

https://bugzilla.gnome.org/show_bug.cgi?id=776966