Store mutable indirections in immutable data with this one weird trick!

[Elronnd]

I think ultimately, the main problem is that d hasn't 
strictly-defined pointer provenance semantics.  So we end up with 
these ad-hoc rules like 'a pure function's result may be 
immutable'.  The C memory model was a great accomplishment on the 
part of boehm, and it should probably not be emulated; 
nevertheless, deviating from it has annoying implications for 
code generation from gdc and ldc.