Allow delete on user-defined value types

We do not allow any operator on user defined value types, but `delete` is special: It is implicitly used when we remove an array element and variables of user-defined value type have a default value - so
```
delete x; // x is of type Type
```
is essentially equivalent to
```
Type y;
x = y;
```
which means we already implicitly use it and it would be confusing to allow re-defining delete to something else.

Start by tracing the compiler handling for operators on user-defined value types and the existing implicit delete behavior when array elements are removed. Confirm the relevant language-semantics and compiler tests, then verify that delete is accepted for these types without becoming redefinable.

Written by the indexing model from the issue text.