Re-add extensions submodule that was accidentally dropped.

diff --git a/ext b/ext
new file mode 160000 (submodule)
index 0000000..4cf18a5
--- /dev/null
+++ b/ext
@@ -0,0 +1 @@
+Subproject commit 4cf18a517a2663e945529a16e5d18e3b676c2687