restructure: move el compiler content into lang/

lang/tests/text/examples/count-substring.el

@@ -0,0 +1,9 @@
+// count-substring.el — non-overlapping substring count.
+fn run_test() -> Int {
+    let s: String = "abc abc abc"
+    return str_count(s, "abc")
+}
+
+fn main() -> Void {
+    println(int_to_str(run_test()))
+}