Cranelift: JITMemoryProvider cannot be implemented because JITMemoryKind is private

[fasterthanlime]

cf. https://docs.rs/cranelift-jit/latest/cranelift_jit/trait.JITMemoryProvider.html

(I did not fill out the bug reporting template because it's it's just a visibility problem.)

My use case was allocating memory with MAP_JIT on macOS to need fewer entitlements.

[fitzgen]

Seems reasonable. Would you like to make a PR fix the visibility?