lib: optimize require() path walking

Remove a speed bump from commit 36777d2 by reusing the result of the
previous stat() system call.  It's a code path that gets called many
thousands of times at startup in most applications so shaving off an
extra system call can have an appreciable impact on startup times.

PR-URL: https://github.com/iojs/io.js/pull/130
Reviewed-by: Chris Dickinson <christopher.s.dickinson@gmail.com>