Support multiple coinbase outputs

[Sjors]

Merge mining and such currently won't work:

https://github.com/Sjors/sv2-tp/blob/36dc8e3652631cb4be813152d7d4cd848d44a3ea/src/sv2/messages.h#L331-L339

This should be easy to fix though.

[xyephy]

@Sjors is this still up for grabs? I can give it a try