change kind_solver option to enum class

[bigfooted]

Proposed Changes

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make kind_solver an enum class

Related Work

Resolve any issues (bug fix or feature request), note any related PRs, or mention interactions with the work of others, if any.
continuing work on making enums into enum classes

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[bigfooted]

I have now added defaults to all getkind_solver switches: the first objective here is to pass the regression tests. It still has to be checked if we need to add solver switches, or throw an error if for some reason we end up in the default switch.

[TobiKattmann]

Thanks for tackling this big chungus

[TobiKattmann]

💐 LGTM. Again, thanks for this