[Feature] Support for synchronous use.

[dabrahams]

I learned recently if you Foundation.Process.waitUntilExit without first draining the standard output and error streams the system can deadlock, because it refuses to allow more data to be streamed out of the child process when its output buffers become full. I don't see any code for handling that situation here.
Here's an example of how it's done. The number of examples of doing this wrong out there are astounding. I know the new hotness is to await everything but I have synchronous code that really does need to block until another process completes, and I'd rather use an off-the-shelf package than repeat my own code. It would be great if this package could provide an answer for these situations, but I don't see one.