Revert "Add an alias for ModeT, the method Int#to_modet and a Int#to_i(base)"

This reverts commit 874b712d48c49601744abf7b3e5cea5eafa586f3.
Revert "Add Dir#mkdir and Dir#rmdir"

This reverts commit 5c278a211d4bebbfb055b3d2ea9362a9dede0979.