Copying large directories uses lots of memory

[jrburke]

@timmywil found that doing a build with an appDir: that had about 450mb of files in it ended up using 600mb before doing the actual dependency tracing.

So there seems to a be a weakness in the copyDir code for node, which does sync file copying via binary writes. This was done because of the ease of dealing with sync file code, but probably need to switch to:

• Use streams instead of sync file IO via writeFileSync
• Or, maybe doing a process.nextTick() to break up the work, allow garbage collector to run?

That second one is a bit hacky, and since it would imply needing a callback into file.copyDir, might as well try the stream approach.