[simpleJIT] Make it possible to dealloc specific functions

- What is the feature or code improvement you would like to do in Cranelift? See title.
- What is the value of adding this in Cranelift? This makes it possible to implement a JIT repl which deallocs any functions which won't be used anymore, for example because it has changed.
- Do you have an implementation plan, and/or ideas for data structures or
algorithms to use?
- Have you considered alternative implementations? If so, how are they better
or worse than your proposal?

See https://zulip-archive.rust-lang.org/131828tcompiler/81358REPLCraneliftbackend.html#182197469 for context.