Fixes #3594 .

Summary/Motivation:

#3594 pointed out that deepcopying a SolverResults object would corrupt the internal data structures. This PR overhauls that class (which has been effectively untouched for more than a decade) to:

• update it to use Python 3.7+ standards (in particular, now relies on dict being implicitly ordered)
• rework & simplify logic for creating, pickling, and deepcopying the class (mostly removed all the custom code that was just not necessary
• make SolverResolts objects comparable
• update testing to reduce reliance on the filesystem
• remove unused/untested code

Changes proposed in this PR:

• (see above)

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