Missing checks in enum value instance
When creating a value instance with `enum`, the compiler doesn't check its `value` in any way, it doesn't check if the prop exists:

```yaml
meta:
  id: inst_value_enum_nonexistent_prop
instances:
  should_fail:
    value: does_not_exist
    enum: fruit
enums:
  fruit:
    1: apple
    2: banana
```

This will normally pass (in the devel Web IDE) and the compiler generates this (for Java):

```java
this.shouldFail = Fruit.byId(doesNotExist());
```

It's interesting that if I modify the `value` to an expression (to `value: does_not_exist + 0`), it will correctly run a precompile check and throw

```
io.kaitai.struct.precompile.FieldNotFoundError: unable to access 'does_not_exist' in inst_value_enum_nonexistent_prop context
```

So it will be probably enough to run the precompile checks even if the value is a plain `Ast.identifier`.