lang: ast: Expr Param and Poly should not have values

Sometimes a recursive call through ExprVar's Value method would hit one
of these and return (nil, nil) which would throw off things.

lang/interpret_test/TestAstFunc2/scope-lambda-printf.txtar

@@ -0,0 +1,12 @@
+-- main.mcl --
+import "fmt"
+$format = "%d"	# should get ignored
+$fn = func($format) {
+	fmt.printf($format, 42)
+}
+test $fn("%s") {}
+# should error at unification if possible, otherwise at runtime
+# TODO: I would expect that if the "%s" and "%d" swapped, that speculatively we
+# would be able to run this at compile time and know the result statically.
+-- OUTPUT --
+# err: errStream: func `printf@??????????` stopped before it was loaded: base kind does not match (Str != Int)