Support caching for realpath, use in module load

This adds support for a cache object to be passed to the
fs.realpath and fs.realpathSync functions.  The Module loader keeps an
object around which caches the resulting realpaths that it looks up in
the process of loading modules.

This means that (at least as a result of loading modules) the same files
and folders are never lstat()ed more than once.  To reset the cache, set
require("module")._realpathCache to an empty object.  To disable the
caching behavior, set it to null.