Add an E' backend

[ozgurakgun]

This will be reasonably straightforward and would allow conjure-oxide to reach full feature coverage very quickly. We output textual essence prime and call savilerow for solving.

[niklasdewally]

Steps to implement

We have a generic solver API which exposes a common interface for all solvers. To add a new backend, one must implement all the functions for the SolverAdaptor interface/trait for it.

Relevant docs:

• Guidelines for writing solver adapters: https://conjure-cp.github.io/conjure-oxide/docs/conjure_oxide/solver/trait.SolverAdaptor.html
• General details about the Solver API: https://conjure-cp.github.io/conjure-oxide/docs/conjure_oxide/solver/index.html