Peekable define function

[bjorn3]

This is the least intrusive way I could find to get the necessary things for bjorn3/rustc_codegen_cranelift#166.

[sunfishcode]

Is it necessary to call the peek_compiled function before doing the self.backend.define_function?

[bjorn3]

No, calling it later simplified it a lot.

[sunfishcode]

Now that the call is at the end, would it be possible for the caller of define_function_peek_compiled to instead call the function itself after define_function_peek_compiled returns, rather than passing in a function to be called at the end?

[bjorn3]

Hadn't thought about that yet. It will require exposing code_size and isa though.

[sunfishcode]

We could have define_function return the code_size instead of (). And Module can provide an accessor to its isa if needed, although do you need the whole isa or just the TargetFrontendConfig? The latter is already available as module.target_config().

[bjorn3]

I need the whole isa to get the encoding_info.

[sunfishcode]

Ok. This PR feels somewhat specialized, and I'd ideally like Cranelift's API to make this kind of thing easier in a more general way, though I don't have any specific ideas at the moment. So let's merge the PR, but keep an eye out for ideas to refactor this :-).