Removing SDL and other misc. UI-based classes
sys/benchmark.d and sys/windows.d seem to be windows-specific, add version(Windows): to them
Assuming 4096 pagesize for OS X (unistd.d doesn't expose a getpagesize call for OS X)
Fix ambiguity between std.ascii.toLower and std.uni.toLower by explicitly invoking std.uni.toLower
OS X doesn't include SIG_BLOCK, make collectSignal in signals.d only available in Linux (and fix up a use of collectSignal in file.d to mark it as linux-specific)
Setting up dub structure