new instmap implementation

[Simon Taylor]

Hi Andrew (and everyone else),

I noticed in the CVS log:
> Log Message:
> Update of alias branch to use new instmap implementation.

Thanks!

Just a few questions:

- This seems to be a fairly major change, are we going to see a diff?
- What impact did this change have on the performance of the compiler?
- What are you using to compile the alias branch?
	 - mercury-latest doesn't compile the alias branch library.

Simon.