[phobos] [D-Programming-Language/phobos] 4f28db: not really pure

[Daniel Murphy]

Could somebody revert this please? (and the similar druntime change)
Weakly pure functions should be able to do anything they like with
data reachable through their parameters, including the 'this'
parameter, as of the new purity rules.
I can't build druntime from head and it seems to be caused by this.

If the rules need to me discussed some more or changed,
dmd+druntime+phobos should at least be compiling while that happens.

I'm fairly sure druntime and phobos are both broken by these changes,
it's just not showing up on the autotester because the dmd tests are
failing as well.