c-rbtree: re-import git-subtree for 'src/c-rbtree'

The subtree was moved from "shared/c-rbtree" to "src/c-rbtree". That confuses
git-subtree. To fix it, reimport the code with

  rm -rf src/c-rbtree
  git commit -a -m 'dummy commit'
  git subtree add --prefix src/c-rbtree git@github.com:c-util/c-rbtree.git 8aa7bd1828eedb19960f9eef98d15543ec9f34eb --squash

Then rebase the result to drop the bogus dummy commit from the history.

To update the library use:

  git subtree pull --prefix src/c-rbtree git@github.com:c-util/c-rbtree.git main --squash

src/c-rbtree/subprojects/c-stdaux

@@ -1 +1 @@
-Subproject commit 346623b40eb8137cae7568a69ee42253ff098ff7
+Subproject commit c5f166d02ff68af5cdcbad1bdcea2cb134e34ce4