Add API for stdout (engOutputBuffer)

[bbarker]

Currently there is no way to retrieve stdout of the MATLAB process; for this we need engOutputBuffer, and possibly some convenience APIs written atop it to make dealing with buffers less burdensome.

[bbarker]

An alternative, possibly more appealing in certain cases anyway (and certainly appearing simpler) is the MATLAB diary, which basically tells MATLAB to log all output to a specified file - I'll explore this soon.

[bbarker]

Confirmed that diary works well for most use cases. Using Path, a couple of example functions would be:

diaryFile :: Engine -> Path b File -> MIO ()
diaryFile e logFile = engineEval e $ "diary " <> (toFilePath logFile)

diaryOn :: Engine -> MIO ()
diaryOn e = engineEval e "diary on"

[bbarker]

Does engOutputBuffer include stderr? What sorts of things are logged to stederr in MATLAB?