Imported c-rbtree code with command:

  git subtree add --prefix shared/c-rbtree git@github.com:c-util/c-rbtree.git bf627e0c32241915108f66ad9738444e4d045b45 --squash

To update the library use:

  git subtree pull --prefix shared/c-rbtree git@github.com:c-util/c-rbtree.git master --squash