Enable Solver construction from borrowed design

[leostimpfle]

Introduces Solver::from_design to construct a solver from a persistent, borrowed &'a Design<'a>.

To keep the diff minimal, I have not removed the existing Solver::new construction which takes ownership of a Design. This is facilitated by a (somewhat hacky) enum that distinguishes the existing owned construction from the new borrowed one:

within/crates/within/src/solver.rs

Lines 585 to 588 in 4c20407

	enum SolverDesignStorage<'a> {
	Owned(Design<'a>),
	Borrowed(&'a Design<'a>),
	}

In case we end up liking the separation of Design and SolverState, we may want to get rid of it. (This will create a large diff because every call-site using Solver::new will have to be updated).