Add cursor fluent-style chaining

[thegamecracks]

This allows chaining execute*, fetch*, start, commit, and rollback methods of Cursor/_CursorWithTransaction and also execute* methods of Connection as such:

async with asqlite.connect(':memory:') as conn:
    row = await conn.executescript(
        'CREATE TABLE test (id INTEGER);'
        'INSERT INTO test VALUES (1);'
    ).execute(
        'SELECT * FROM test;'
    ).fetchone()

    async with conn.execute(
            'INSERT INTO test VALUES (2);').execute(
            'SELECT * FROM test;') as c:
        rows = await c.fetchall()

This is related to issue #5 which attempted chaining an execute and fetchone, resulting an an exception due to trying to use a cursor created by the worker thread in the main thread. This PR does not support their syntax however since they are awaiting c.execute(...) by itself, which would still return an sqlite3.Cursor here.

[Rapptz]

I'm not really big on the implementation here. It feels rather inefficient and bolted on.

The Python ecosystem as a whole is probably unfamiliar with the concept of async chaining like you're doing here. It's definitely foreign and blurs the line on where await should be required so I'm not super big on it. In Rust this is less of a problem since they use .await syntax but in Python chaining like this is verbose.

There's probably some ergonomic tradeoff to be made here but I'm not super into this implementation as it stands.