Fix memory leaks & remove a dubious cast

[adbancroft]

1. OverrideableProxiedArduinoFake_t::getFake uses a static_cast to potentially cross cast a fakeit::Mock. E.g. from fakeit::Mock<Stream> to fakeit::Mock<Print>. This isn't valid since fakeit::Mock is a template class and there is no relationship between mocked types. I.e. even though Stream is derived from Print, fakeit::Mock<Stream> is not derived from fakeit::Mock<Print> (dynamic_cast would return nullptr). Fixed using an inherited interface: IFake
2. ArduinoFakeMock was returning a heap allocated object, resulting in memory leaks. Changed to return a std::shared_ptr