Move the integrations in a subfolder and rename binder

[Sytten]

Now with diesel we will have 4 integrations, they should probably be put in a subfolder.
I think we should consider deprecating sea-query-binder and renaming it to sea-query-sqlx.

[billy1624]

Hey @Sytten, I agreed, that would be more explicit and clean.

[tyt2y3]

I've published https://crates.io/crates/sea-query-sqlx

I don't think it's necessary to put them into sub-directory, I think it's managable now.