Re-export `lightning_types` in top-level `lightning` modules

Re-export `lightning_types` in top-level `lightning` modules

Since we now have many types in one place, it makes sense to export
them in that place. Further, doing so finally somewhat starts to
reduce our `lightning::ln` module size, which historically is the
dumping ground for everything when most things really should be
top-level modules in `lightning`.

Here we take a step in the right direction by exporting
`lightning_types` as `lightning::types` and encouraging users to
use those paths directly rather than the ones in `lightning::ln`.