Use Cranelift as a backend.

[nbp]

Cranelift (previously named Cretonne) is far from being ideal as the Cranelift IR does not provide the features needed to express versioning needs, and using it as a backend implies having an additional intermediate representation.

However, Cranelift does have more stable code base and would remove a good chunk of work from HolyJIT early days. Thus, it makes sense to base HolyJIT on Cranelift for the early days of HolyJIT until it becomes a pain point.