In new IO library, improve representation of FileMode

[catamorphism]

See FIXME (formerly XXX) -- though I think the current data structure might be as good as we can get. (It could instead be a pair of "what to do if the file doesn't exist / what to do if the file exists", but we want to rule out some possible combinations.)

[olsonjeffery]

I'm using this as part of #8655 .. I don't think it's that bad, tbh. But, yeah. open to suggestions.

You don't have to squint too hard to realize that this abstraction is derived from unix file mode flags passed to open(2). And our initial impl atop libuv also uses the same flags. So it's working pretty well, there. But will that always be the case for other underlying io frameworks?

[alexcrichton]

Closing in favor of #2623, that bug is tagged and nominated, and it has essentially the same conclusion as this bug.