Allow the evaluate end-point to separate output from compiler

vs. program.

Specifying separate_output=1 in an evaluate request will mean the return
value is then `{ "rustc": <output of the compiler>, "program": <output
of the program> }`.
I believe that's how one handles an empty `$3` in a reliable way... but I'm just cargo-culting it from other places, so it could be complete nonsense.