compiler: implement casting named structs and pointers to them

testdata/structs.txt

@@ -0,0 +1,11 @@
+test0
+test1 1
+test2 1 2
+test3 1 2 3
+test4 1 2 3 4
+test4b 1 2 3 4
+test4bp 1 2 3 4
+test5 1 2 3
+test6 foo 3 5
+test7 (0:nil) 8
+test8 2 3 12 13 6