Schema management

For comment:
* Add an .edsl reader interface (blocking/ not async)
* Add validation for .edsl files (blocking/ not async)
* Add a direct `migrate` or `apply_schema` interface (blocking/ not async)
* Add a direct `export` interface, (also blocking) perhaps something like:
> from pathlib import Path
> from typing import Union
> export(db: str, path: Path, tables: Union[set, list, tuple]) # what other options?

My expectation is that these are not likely to and should not be run in an async context, but maybe I'm just not aware of the use case. I would be happy to work on this piece of the project.