Support larger # of source files

[jameskoch]

Larger projects need an alternative to supplying individual source file names on the command line, especially on Windows where there's a fundamental limit of ~32k characters for CreateProcess arguments.

If I were using javac or scalac directly I'd use @argfiles. I haven't tried using argfiles w/ the -S pass-through, but I'm assuming that would circumvent a ton of meaningful zinc functionality. If that's true, then it seems like zinc could support argfiles itself. I'm happy to do the leg work here if this seems like a sensible approach.

I haven't worked w/ -nailed; I'm not sure if that suffers from the same limitation as direct zinc usage.

[pvlugter]

That would be useful. Contribution would be very appreciated. Thanks!