[RFC] Revert "Port mch_isdir to libuv."

Revert commit 8bb672e6a088eb77d98d182bd46551986b52eb49 which caused a memory
leak error. This commit was merged from pr #290 and wasn't detected because
travis wasn't running the tests on valgrind(fixed now).

Valgrind slows test runs by a factor of 20, but it's worth because memory leak
errors are sometimes a pain to find.