Avoid slow `abs_dirname()` by loading `realpath` extension

This speeds up every `rbenv` invocation significantly.

libexec/rbenv

@@ -12,6 +12,12 @@ if [ -n "$RBENV_DEBUG" ]; then
   set -x
 fi
 
+if enable -f "${0%/*}"/../libexec/rbenv-realpath.dylib realpath 2>/dev/null; then
+  abs_dirname() {
+    local path="$(realpath "$1")"
+    echo "${path%/*}"
+  }
+else
 READLINK=$(type -p greadlink readlink | head -1)
 if [ -z "$READLINK" ]; then
   echo "rbenv: cannot find readlink - are you missing GNU coreutils?" >&2
@@ -35,6 +41,7 @@ abs_dirname() {
   pwd
   cd "$cwd"
 }
+fi
 
 if [ -z "${RBENV_ROOT}" ]; then
   RBENV_ROOT="${HOME}/.rbenv"