Remove `SolutionVariables`, add ConstrainedSubst analogous to Chalk#8387

Merged

bors[bot] merged 1 commit intorust-lang:masterfrom flodiebold:remove-solution-variables

Apr 6, 2021

+53-33

Commits on Apr 6, 2021

Remove SolutionVariables, add ConstrainedSubst analogous to Chalk
flodiebold
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