C-API

tests: basic kani harnesses for `Var` and `Lit` #166

Job	Run time
Build and test (ubuntu-latest)	52s
Test generated C header	9s
Build and test (macos-latest)	46s
Ensure C-API crate version is in sync	2s
Build and test (windows-latest)	4m 19s
	6m 8s

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