Standalone AA implementation ready for review

[bearophile]

Andrei Alexandrescu:

> I think the built-in associative array must allow non-constant 
> keys. As long as there's no memory safety issue, the AA should 
> work with types that the user doesn't change for comparison 
> purposes but can otherwise modify.
>
> A practical matter is that if we introduce this restriction 
> we'll break a ton of code.

Related to this I have one E.R.:
http://d.puremagic.com/issues/show_bug.cgi?id=6253

Opinions welcome :-)

Bye,
bearophile