Requiring weak purity for opAssign, postblit

[bearophile]

Tomek Sowiński:

> 2. Functions ought to be weakly pure by default (please don't dismiss as
> bikeshedding). It's the most common case by a large margin.

This is right. I'd like functions to be pure on default and use an attribute like @outer() when whey aren't pure because they use mutables from outer scopes (in the () of outer you optionally list all the in/out/inout mutables from the outer scope). But probably this will not change now.

Bye,
bearophile