instance overlap in 6.6 candidate

[Simon Peyton-Jones]

| What are the disadvantages of compiling every library
| with -fallow-overlapping-instances?

It's not H98, so it has to be an option, enabled by a flag.  Of course
you are free to put the flag at the top of every module.

Sergey's point is that the libraries are pre-compiled, so you can't add
flags to them. 

The old model was that every instance is potentially overlappable; and
you only need the flag when you *use* the instances.  But people
complained that the clients of their library should not need to know
"import Foogle and use -fallow-overlapping-instances".

Simon