Runner IntoIterator support

[superstator]

• adds an impl of IntoIterator for Runner, as a third way to run migrations beyond run() and run_async()
• moves the connection argument from run() to Runner::new()
• refactors some migration code to reduce duplication between sync/async implementations and make handling of Target::Fake more consistent

[jxs]

Hi again, I am sorry for the delay and thanks for your work with this! I see a problem with this implementation imho, having Runner become Runner<'a, C> heavily limits what you can do with it, basically Runner can't leave the scope where it was created, see here for an example.

Even if it might not mean actual practical limitations as there might not be a use case for a user wanting to move Runner across scopes it might exist in the future, and I feel like limiting this for one function is not worth the tradeoff. I checked if we could take ownership into Connection but the first crate checked, rusqlite doesn't impl Clone for Connection so it's a no go.

The solution I see is what we discussed on #119 (comment), having an iter(&self, &mut C) -> Iter<'a, 'b, R, C> which can impl std::iter::Iterator which also impl's std::iter::IntoIterator. I understand it's not conventional, but I am more comfortable with it than limiting Runner to become Runner<'a, C> wdyt?

Thanks!