Project Restructuring: Further modularize codebase

[tnull]

As discussed out of band it would be very useful if we could further modularize and expose the codebase at each of the individual levels.

For example, we want to allow third parties to reuse our message types/serialization, or, just a specific LSPS implementation, without the need to use the full LiquidityManager and even without relying on LDK-specific *Manager types. To this end we'll need to make sure that not too many LDK types (in particular ChannelManager/PeerManager leak into the 'inner' part of the LSPS implementations.