test for fully unbound case

testsuite/query-fully-unbound/main.tri

···
+rule always(_)
+
+proc main!() {
+  let mut n = 0
+  for always(1) {
+    n += 1
+  }
+  for always(^n) {
+    n += 1
+  }
+  for always(r) {
+    exit 1
+  }
+  assert n == 2
+}

trilogy-llvm/src/expression/mod.rs

···
         name: &str,
     ) -> Option<PointerValue<'ctx>> {
         match &application.function.value {
+            Value::Builtin(Builtin::Pin) => {
+                return self.compile_expression(&application.argument, name);
+            }
             Value::Builtin(builtin) if builtin.is_unary() => {
                 return self.compile_apply_unary(*builtin, &application.argument, name);
             }