revert recent regex-syntax interval set optimizations #1102
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Specifically, this PR: #1051
Multiple bugs were caught by OSS-fuzz. Ideally we'd just fix them, but this code is extremely subtle and we'll need to make these changes more carefully (if at all).
cc @Licheam if you want to take a crack at fixing these. I think we'll maybe want to land one optimization at a time perhaps? And I'll have to insist on better unit test coverage as well.