evaluate the function position correctly

* module/language/elisp/compile-tree-il (compile-pair): Use `function'
  to perform functional evaluation. Previously, if the operator of a
  compound form was not a symbol, the operator would be evaluated as a
  normal expression. This happened to work only because there is a
  `lambda' macro. The compiler will now signal an error if the operator
  is neither a function name nor a lambda expression.

* test-suite/tests/elisp-compiler.test ("Lambda Expressions")["optional
  argument"]: Remove an erroneous use of `function' in the function
  position.

module/language/elisp/compile-tree-il.scm

@@ -841,11 +841,7 @@
            (compile-expr (apply macro-function arguments))))
      (else
       (make-application loc
-                        (if (symbol? operator)
-                            (reference-variable loc
-                                                operator
-                                                function-slot)
-                            (compile-expr operator))
+                        (compile-expr `(function ,operator))
                         (map compile-expr arguments))))))
 
 ;;; Compile a symbol expression.  This is a variable reference or maybe

test-suite/tests/elisp-compiler.test

@@ -379,7 +379,7 @@
     ((lambda (a b c) c) 1 2 3))
 
   (pass-if-equal "optional argument" 3
-    ((function (lambda (a &optional b c) c)) 1 2 3))
+    ((lambda (a &optional b c) c) 1 2 3))
   (pass-if-equal "optional missing" nil-value
     ((lambda (&optional a) a)))