Homogenize logging outputs and clean up error types

[tnull]

Currently the wording and degree of detail can vary a bit depending on the logged message.

We should go through each of them and make sure all logged messages follow a similar structure. Also, we might consider introducing further grouping / log levels when, e.g., handling events.

We should also clean up the error types and, instead of wrapping all upstream errors, only expose sensible easy-to-handle types to the user.

[tnull]

While we're at it, we should also clean up the naming scheme. In particular we should make a clear decision if we want to keep the LdkLite prefix for the objects or not.

[tnull]

Now addressed (most of) this directly in #11.