- An argument that we didn't use was removed from the JSFinalizeCallback
  signature

- JS_SetLocaleCallbacks API changed (is JSRuntime coming back again?)

- Giving a JSVERSION to a compartment or CompileOptions is not supported
  anymore

- Use JS::CurrentThreadIsHeapCollecting() instead of our hacky workaround
  in GC

- JS::PromiseRejectionHandlingState now is in the JS namespace

- JS::Value is now incompatible with C-linkage, so any function returning
  one must be moved outside G_BEGIN_DECLS/G_END_DECLS

- JSPROP_SHARED is gone

- Defining a property with getter and setter now doesn't take a JS::Value

See: #161