committing transaction returns lmdb::Error instead of StoreError

[mykmelez]

The recent changes in #101 directly expose the lmdb::RoTransaction and lmdb::RwTransaction types instead of wrapping them in Reader and Writer types when calling Rkv.read() and Rkv.write().

This has the side-effect of also exposing the lmdb::Error type when calling R[o|w]Transaction.commit(), which was previously converted into a StoreError by the Reader.commit() and Writer.commit() functions.

And that's inconsistent with most of the other functions in the public API, including Environment.begin_r[o|w]_transaction() and the various Store::[Single|Multi|etc.]Store functions, which all wrap an lmdb::Error in a StoreError::LmdbError.

It's also obviously inconsistent with any other function that returns another type of StoreError, and it means that consumers of rkv need to handle both the StoreError type and the underlying lmdb::Error type.

We should ensure that the public API returns StoreError consistently to indicate failure.