Create separate posix module for Unix based platforms

[slide]

We currently just remove builtins['nt'] and assign builtins['posix'] to be the nt module. There is some overlap in functionality, but there are some things that would be much easier to implement in separate modules. Another option would be to use a #if check on Mono builds and have to separate releases, one for Windows and one for Unix based systems. This also doesn't even bring in the concept of running on OSX, which may have some additional changes.

[slozier]

Separate modules sounds fine as long as we can share the common code.