recursive equal, and firstDifference functions

[timotheecour]

> I think I understand what you are after and I like the idea. 
> I've got a few:
> -typesDeepEqual
> -typesDeepCmp
> -deepHash


approxEqualRecurse could also be useful for numerical 
applications (same as equalRecurse, except uses approxEqual for 
floating point comparisons)

> https://github.com/patefacio/d-help/blob/master/d-help/opmix/mix.d

Thanks for the link, I'll take a look.