Make Solver Send + Sync

[elbaro]

This PR changes

    objective: Rc<RefCell<Row>>,
    artificial: Option<Rc<RefCell<Row>>>,

to

    objective: Row,
    artificial: Option<Row>,

I am a little unconfident of this PR because the conversion was too straightforward.

[orhun]

Hey, thanks for the PR.
The changes look okay but I haven't checked the details (performance impact, safety, etc.)
I'm just wondering about your use case for this :)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 57.14286% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.26%. Comparing base (4aad6db) to head (4ca11cd).

Files with missing lines	Patch %	Lines
src/solver.rs	57.14%	9 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #50   +/-   ##
=======================================
  Coverage   76.26%   76.26%           
=======================================
  Files           8        8           
  Lines        1230     1230           
=======================================
  Hits          938      938           
  Misses        292      292           

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[elbaro]

I use Send + Sync for parallel rendering.

let widgets: Vec<Widget> = ..;
widgets.par_iter().map(|widget| widget.render(&solver)); // rayon

If Solver is Send + Sync, &solver can be passed to N threads and read in parallel.
Each widget has a different set of variables, so it's hard to prepare Vec<WidgetLayoutValues> in advance.