Massive slowdown in mwc-random after switching to use of primitive package

[Simon Peyton-Jones]

| The head has -dsuppress-coercions which omits coercion terms when pretty
| printing Core. It would be easy to backport that to 6.12.

The HEAD also has a coercion optimiser that dramatically shrinks some large coercion terms.

| > I might revert both mwc-random and statistics back to using plain ST for
| now, then, as having them run slightly faster than 1% of their former speed
| is a bit painful :-(
| 
| I fear that's the only sensible solution. I wouldn't throw away your current
| code, though, as 6.14 won't have these problems (hopefully).

Yes, I hope that's possible because I really really don't want to have to dig into 6.12's inliner.  Partly we'd then need to make yet another release, and partly it's just delicate -- that's why I redid it for 6.14.  It's very helpful to have the test, though, so I can ensure 6.14 stays good. Thanks for that

Simon