Move the public channel state API into a new module

Move the public channel state API into a new module

Our "what is the channel's current state" structs have slowly
grown to be rather nontrivial, and now include eight structs with
many fields. Thus, it makes sense to pull them out of
`ln::channelmanager` and into their own module.

This also makes things easier for the C bindings which don't
support `pub use` from a private module.