Help avoiding usage of unserializable parameters in proxy services.

[jalooc]

The proxy services functioning is based on postMessage(), which only works with serializable data. It would be nice touch to prevent devs from passing any non-serializable data, like functions or classes (e.g. CustomError thrown in background script will fail the instanceof CustomError check in a content script), to avoid wtf moments when it doesn't reach the other end of the communication channel, being silently filtered out. I thought this could be done with TS by making sure the methods of the object passed to defineProxyService don't have parameters or return values of illegal types.

[aklinker1]

Fixed by #79, released in https://github.com/aklinker1/webext-core/releases/tag/messaging-v2.0.3