Disallow side effects in assert

[bearophile]

Daniel Murphy:

> Enough to tell it _might_ have a side effect, not enough to 
> know it _will_. The number of false positives would make it 
> near useless.

How many impure functions do you call in your asserts?
In D we have a purity system, and now it works well enough. It's 
a good idea to actually start use it, for asserts, contracts, 
parallelism, etc.

Bye,
bearophile