- Added OpCodes melt/freeze which are how deep-copying of vars will be done. ie `VarSet(Melt(Value))` and `Freeze(VarGet(Var))`
- melt suspends upon encountering a placeholder
- freeze errors on encountering a placeholder. This may change depending on whether the Verse-Verifier decides to allow this or not.
- note: We use a VCell header bit to identify types which are deeply mutable and thus require melt/freeze operations (this may be moved into these types emergent types)
- Re-worked VMap classes so now majority of code lives inside VMapBase via templating
#rb saam.barati
[CL 30688090 by jared cotton in ue5-main branch]