Requiring weak purity for opAssign, postblit

[dsimcha]

== Quote from bearophile (bearophileHUGS at lycos.com)'s article
> dsimcha:
> > Does anyone see a problem with requiring opAssign and postblits to be
> > weakly pure to solve this problem?
> Time ago I have even suggested contracts (pre/post/invariants) to be pure (as
Design by Contract asks), or at least class invariants to be const on default.
> Bye,
> bearophile

Yeah, this is a good idea iff we fix the template pure issue.