Make parseUUID pure

http://dlang.org/phobos/std_uuid.html#parseUUID :

> BUGS:
> Could be pure, but this depends on parse!(string, 16).

I was reading this, and I remembered that recently, `parse!(string, 16)` was made pure (and safe), so I changed the code accordingly. It now only relies on inference (and a `@trusted` exception builder).

Tagging @jpf91 .