These hooks are going away in SpiderMonkey 60, so we have to do something
different. Instead of intercepting the getProperty/setProperty
operations, we lazily define a JS property for each GObject property or
field accessed in the resolve hook.

This uses the same native-accessor-with-private-data API used in
gi/boxed.cpp. Two separate sets of accessors are used for properties and
fields, which makes them a bit simpler than the previous code.

One behaviour change is that we now can't distinguish whether we're
setting a readonly property in strict mode or not. Now we always throw
when setting a readonly property; in effect, we treat the property set as
if it were always in strict mode. ES6 modules are always treated as
strict mode, so this is a change that client code will have to go through
anyway when using ES6 modules.

Closes: #160