switch drcontext from void* to a specific type to catch more errors at compile time

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on May 30, 2013 11:22:14_

On Thu, May 30, 2013 at 11:11 AM, a user wrote:
> It's a really silly mistake, I passed drcontext as the first argument to instrlist_meta_preinsert and that's the root cause. The compiler didn't give any warning as drcontext is void*.

Perhaps we should use a specific type for drcontext to help catch this kind of error?

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1173_