RpcMapper mostly useless

[AnotherDaniel]

I find the current iteration of the RpcMapper helper functions to be mostly useless; more precisely, they will only work in the special case where there's Any objects used to wrap UMessage payload content. While this might be a default case, it does forgo the opportunity to take into consideration the UPayload.type, and extract content accordingly.

I am working on an improved version, in conjunction with the up-subscription effort, this will result in an update/extension to RpcMapper sometimes in the coming weeks. This issue is to let you guys know that I'm touching things in this area, just in case someone else feels the same pain...

[AnotherDaniel]

Done with #108