Consider exporting commonly used Python classes from the main fdb module

I use lots of Type Annotations as formalized by [PEP 484](https://www.python.org/dev/peps/pep-0484/). They have been available since Python 3.5.

However, since the classes are only defined in sub-modules like `subspace_impl`, `directory_impl`, `impl`, `tuple`, etc, many of the annotations end up more verbose than necessary. For example:

```python
@fdb.transactional
def tr_delete(
tr: fdb.impl.Transaction, triple: BasicTriple, subspace: fdb.subspace_impl.Subspace
):
```

This is definitely a minor change, and could be worked around by writing `from fdb.impl import Transaction` or similar.

Review the main fdb module alongside the referenced submodules: subspace_impl, directory_impl, impl, and tuple. Identify which commonly used classes should be available from fdb for shorter PEP 484 annotations, and check the existing Python API tests or import conventions. Done means the selected classes can be imported directly from fdb.

Written by the indexing model from the issue text.