Hide internal modules

[wenkokke]

We would like to hide the internal modules from the public API. After discussion, there are three options:

1. Publicly export the internal modules under the Database.LSMTree.Internal namespace.
2. Migrate the internal modules to a private sub-library.
3. Migrate all modules to a private sub-library,
   then create modules in the public library that re-export the public modules.

Our consensus is that we should not make any of these changes now, since all of these affect too many files, but that we should settle on which of the three options we should use before we cut our first release.

Overall, we felt positive about options (2) or (3), but were slightly divided between the two options. Option (3) is likely easier for developers and maintainers, but there was a fear that it might have a slightly higher performance impact. Hence, if we choose option (2) or (3) we should benchmark the change to see if there is any performance impact from the potential loss of inlining that might result from this rewrite.