Consider making `CompileResult` members private.

[otrho]

When reviewing #2472 I noticed it's possible to accidentally make an invalid CompileResult, where the author intended to return an 'ok' but set value to None.

We should make it mandatory to use CompileResult::ok() and CompileResult::err() to avoid this sort of thing. It would also required making a bunch of access methods for pulling the value or errors and/or warnings out which may turn out to be quite a pervasive change.