[Issue 447] frontend: writeable string constants

[Walter Bright]

Thomas Kuehne wrote:
> Sure, it is a lot of work and the current solution is only a start.
> (for the time beeing you could replace "const_cast<char *>" with "mem.strdup")
> "Pritty sure" is nice but having as much as possible automatic
> detections is more important to me in the long run.

I've fixed a lot of bugs in dmd over the years (you know that as well as 
anyone!). Not one of them would have been found by const-correctness. 
I've used const-correctness in the past, and it never found any bugs 
then, either.

My bugs tend to be cases I hadn't thought of, i.e. unexpected 
interactions between features.

> <rant>
> Trying to clean up the frontend for 64bit proved surpisingly hard.
> 64bit builds with gcc and icc show different behaviours.
> Seemingly simple test cases like
> http://dstress.kuehne.cn/run/l/large_id_01_D.d and
> http://dstress.kuehne.cn/compile/template_class_08.d
> fail for 64bit but pass for 32bit builds. etc.
> </rant>

I'm very interested in folding in the changes necessary to fix this. 
I've already folded in the ones listed in the bug reports here (they'll 
be in the next update).