Reject inedexed zero sized type

### What is wrong?
Currently, it is allowed to mark a zero-sized type field as an indexed field, but this doesn't make sense because a zero-sized type will never be encoded.

e.g.
```
event MyEvent:
idx x: ()
```
should be rejected.

Start by locating the compiler validation for indexed fields in event declarations and how zero-sized types such as () are represented. Reproduce the example with `idx x: ()`, then make the compiler reject it with an appropriate diagnostic and verify that valid indexed fields still compile.

Written by the indexing model from the issue text.