POSIX and Time plugins

[Brian Rice]

src/plugins now has two more entries: time/ and posix/. The time  
plugin just exposes ANSI definitions in a minimal way, and the POSIX  
plugin will contain posix-specific API's but for now just has the  
minimum file-system manipulation primitives. I'll grow these  
gradually (not sure where I'll stop, but there are some API's that  
obviously just aren't worthwhile).

I've also added loadPlugin: to extlib.slate (requires running the  
post-bootstrap to load it in or copy-paste from the file), which  
basically calls load: 'src/plugin/X/X.slate' as a convenience method.

Finally, Platform run: is now used to invoke "make VM=" for the  
output VM generated in the standard bootstrap process via  
fullCleanBootstrap. If it fails, it won't hold anything else up (run:  
just returns the exit code as a Boolean).

--
-Brian