[phobos] Pureness of enforce()

[Lars Tandle Kyllingstad]

enforce(x) is basically a substitute for

  if (x) throw new Exception;

which doesn't violate the purity rules.  It is not possible to annotate
enforce() with 'pure', however, because it takes a lazy parameter, which
is just shorthand for a (possibly impure) delegate.

enforce() is used virtually everywhere in Phobos, and this means that a
lot of functions that could otherwise be marked as pure, currently
can't.

One example is std.conv.to(), which should definitely be pure.  (In
fact, I tried marking it as such, which is what got me thinking about
this in the first place.)

What to do?

-Lars