Stringified interface

[swfsql]

The interface is currently using stringified objects instead of the objects themselves.
I think that this is less efficient, since this includes some serialization/deserialization steps.
Besides that, the generated documents are more clear if the interfaces use structural types, instead of using String.

I'd prefer if callback functions parameters were not stringfied function names, but also references to the javascript functions themselves, as it would enable some improvements on the callback data flow.

[swfsql]

Currently, as the interface is largely stringified, direct JsValue (with wasm_bindgen) translation of structs are being replaced by derivations from serde, as they are more malleable anyway.