Requiring weak purity for opAssign, postblit

[dsimcha]

I've been thinking about adding purity to various parts of Phobos and one
thing that limits this in the case of containers is templated containers,
arrays, etc. that may be instantiated with a user-defined type.  In theory,
even assigning/copying a user-defined type could be impure.  Does anyone see a
problem with requiring opAssign and postblits to be weakly pure to solve this
problem?