cgo: implement struct types

Not complete: packed structs are treated as regular structs.

testdata/cgo/main.c

@@ -8,6 +8,7 @@ double globalDouble = 3.2;
 _Complex float globalComplexFloat = 4.1+3.3i;
 _Complex double globalComplexDouble = 4.2+3.4i;
 _Complex double globalComplexLongDouble = 4.3+3.5i;
+collection_t globalStruct = {256, -123456, 3.14};
 
 int fortytwo() {
 	return 42;