Avoid intermediate mapping in ZincWorkerImpl

[lolgab]

Profiled Mill compiling Mill itself and 0.07% of the time was spent in mapping the result of compileMixed0 to CompilationResult.
This changes the compileInternal method to already use the right types so less conversions are needed.

[lefou]

The change looks good. But I don't think we save anything here, as the measured wrapping "overhead" comes most likely from the PathRef construction, which we now do in another place.

[lolgab]

The change looks good. But I don't think we save anything here, as the measured wrapping "overhead" comes most likely from the PathRef construction, which we now do in another place.

The gain is very small but doesn't make the code less readable so totally worth it for me.