Ensuring the manifest endpoint listing is tested

[decentral1se]

Following #241 there should be a way to test the manifest exposes what we think is exposed. Somehow matching up all the registered handlers against what is hard-coded in the manifest JSON blob should do it?

[decentral1se]

I'm actually a bit hesitant to create more intricate unit tests for things unit tests are not specifically good for (e.g. tracking down deadlocks). I think this is one of those things. So, instead, I'll optimise for docs and people being able to quickly scan / grok what is there, what isn't and hopefully dipping a toe into using a MUXRPC client. The issues / fixes should come from there. Did this: https://dev.scuttlebutt.nz/#/golang/?id=muxrpc-support-table