Make ManyChannelMonitor Keys an associated type

Make ManyChannelMonitor Keys an associated type

Instead of using a raw generic type, an associted type allows us
to have explicit docs on the type, which is nice. More importantly,
however, our automated bindings generator knows how to read
associated types but not raw generics.

Also, our bindings generator expects things which are referenced to
have already been defined, so we move ManyChannelMonitor below the
ChannelMonitor definition.