Add enums

Could be nice to have user-defined enums
```
enum RESULT {
   SUCCESS=0,
   FAILURE=1,
   UNKNOWN=2
}
```

That can be used inside switch-case as literals:
```
switch (result) {
   case SUCCESS: ...
   case FAILURE: ....
   case UNKNOWN: ...
}
```


And as parameters to functions
```
void foo(RESULT r) {
   if (r == SUCCESS) ...
}
```