Preprocessing can be refactored into form-level processing of integrals and expression-level preprocessing usable without a form

Currently we have preprocess(form, ...) and preprocess_expression(expr, ...) which is used only by uflacs, but they both do mostly the same and can be merged with some refactoring. At the same time, the form signature should be computed from the form resulting from a canonical merging of integrals over subdomains. Currently this merging happens in the translation to integral_data and is thus not seen in preprocessed_form.