Improve interface for adding new backends

[lopeetall]

The backends we have available now were handwritten from scratch, but a lot of the code between the two is similar. Perhaps using Rust traits we can provide an interface that makes it easier for us or any user to include a new backend.

Some things to include in the trait:

• functions for prove, verify, (and setup if applicable). These often require initializing some structs and performing some preprocessing as required by the backend.
• bindings for gadgets, e.g. range 32 x; in vamp-ir calls some built-in range gadget in the backend. Related to Synthesize with backend custom features #48