$is_const is deprecated in favour of @is_const based on $defined.

`$foo` variables could be assigned non-compile time values.
`$foo[0] = ...` was incorrectly requiring that the assigned values were compile time constants.

test/test_suite/compile_time/var_must_be_ct.c3

@@ -0,0 +1,20 @@
+fn void test1()
+{
+	int a;
+	var $x;
+	$x = a; // #error: only assign constants to a compile time
+}
+fn void test2()
+{
+	int a;
+	int[2] $x = { 1, 2 };
+	$x[0] = a; // #error: argument must be a constant value
+}
+
+int g;
+fn void test3()
+{
+	int*[2] $x = { &g + 1, null };
+	int* $y = &g + 1;
+	$x[0] = &g + 1;
+}