Remove references from __assigned_ordinal objects

[DanielThomas]

Hollow's Memoized* collections and user implemented types that use the __assigned_ordinal optimization don't require their references once the object has been fed to the write state.

In my case, I implemented a Memoized interface that exposes a method I can call to drop references after the class is assigned an ordinal, but it occurred to me this is an optimization the producer could optionally take care of - perhaps collection specific clear/trim, nulling all reference fields of a user provided instance, etc.

[DanielThomas]

Occurs to me that you'd have to be careful w/ concurrency when doing this optimization, so maybe it's just best left for advanced cases - perhaps a post-producer process that identifies opportunities for assigned ordinals or inlining?