Be sure to UNPACK the size of an array

The code before did actually unpack the size, I think, but it
wasn't very clear, and it's better to do it explicitly
Make absolutely sure that 'done' and 'safeIndex' are strict in the index

This is just to make sure that there is no redundant boxing.
For safeIndex, for example, the error path doesn't evaluate the
index, so it may be passed boxed unless safeIndex is inlined bodily,
which I don't want to rely on.
Be less verbose when printing Names when we don't know what's in scope

Previously we always printed qualified names, but that makes a lot of debug or
warning output very verbose. So now we only print qualified names with -dppr-debug.

Civilised output (from pukka error messages, with the environment available) is
unaffected
Don't eta-expand PAPs (fixes Trac #9020)

See Note [Do not eta-expand PAPs] in SimplUtils.  This has a tremendously
good effect on compile times for some simple benchmarks.

The test is now where it belongs, in perf/compiler/T9020 (instead of simpl015).

I did a nofib run and got essentially zero change except for cacheprof which
gets 4% more allocation.  I investigated.  Turns out that we have

    instance PP Reg where
       pp ppm ST_0 = "%st"
       pp ppm ST_1 = "%st(1)"
       pp ppm ST_2 = "%st(2)"
       pp ppm ST_3 = "%st(3)"
       pp ppm ST_4 = "%st(4)"
       pp ppm ST_5 = "%st(5)"
       pp ppm ST_6 = "%st(6)"
       pp ppm ST_7 = "%st(7)"
       pp ppm r    = "%" ++ map toLower (show r)

That (map toLower (show r) does a lot of map/toLowers.  But if we inline show
we get something like

       pp ppm ST_0 = "%st"
       pp ppm ST_1 = "%st(1)"
       pp ppm ST_2 = "%st(2)"
       pp ppm ST_3 = "%st(3)"
       pp ppm ST_4 = "%st(4)"
       pp ppm ST_5 = "%st(5)"
       pp ppm ST_6 = "%st(6)"
       pp ppm ST_7 = "%st(7)"
       pp ppm EAX  = map toLower (show EAX)
       pp ppm EBX  = map toLower (show EBX)
       ...etc...

and all those map/toLower calls can now be floated to top level.
This gives a 4% decrease in allocation.  But it depends on inlining
a pretty big 'show' function.

With this new patch we get slightly better eta-expansion, which makes
a function look slightly bigger, which just stops it being inlined.
The previous behaviour was luck, so I'm not going to worry about
losing it.

I've added some notes to nofib/Simon-nofib-notes