Check for re-entry in constructors

**Issue by [error454](https://github.com/error454)**
_Friday Jul 05, 2019 at 22:39 GMT_
_Originally opened as https://github.com/SkookumScript/SkookumScript-Plugin/issues/9_

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**Is your feature request related to a problem? Please describe.**
I wrote a variable initializer in `BaseArconiaPawn`s constructor as:
`@arconian_owner : BaseArconiaPawn!`

I fully intended on that being:
`@arconian_owner : BaseArconiaPawn!null`

When I next hit play I got a stack overflow.

**Describe the solution you'd like**
Throw an assert with a relevant warning if this situation arises. Or better yet make it a compiler error.