This function creates a new directory in the FST, called *folder*. The *folder* name can be a relative or absolute path (starting with "/"). This function handles recursive mkdir calls. If you specify a path with more than one missing dir (eg. "/stuff/home/this/that"), all directories will be created up to the last one specified.
for speed reason. Once the dir is created, it it automatically set to be the current one. You don't need an afc_fstree_cd()