PS I realised that this is more like a singleton initialiser, not a 
constructor.  Since a constructor could be solved by creating the state 
within the init fn call.

>
> But when I go back to the original 'crux' line of code, first it 
> complains it has to be constant because of 'type', but const cant be 
> re-assigned...  So now the problem is probably outside of Zig language 
> support, since this suddenly feels like a missing constructor, or 
> attempt at lazy/lateinit of a comptime/const.