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This is a hopefully temporary measure until the new SRT design is
implemeented.

Improvements:

 - we now turn off the timer signal in the non-threaded RTS after
   idleGCDelay.  This should make the xmonad users on #5991 happy.

 - we now turn off the timer signal after idleGCDelay even if the
   idle GC is disabled with +RTS -I0.

 - we now do *not* turn off the timer when profiling.

 - more comments to explain the meaning of the various ACTIVITY_*
   values

This reverts commit dd24d6bc.

This attempt to fix the problem was misguided: the program might be
stuck in a foreign call rather than awaitEvent(), and then the timer
signal will never get disabled.  The only way to turn off the timer
signal in this case is in the timer interrupt handler itself.

We only warn when the method is used, not when it is defined as part
of an instance.

Object sizes still unchanged.

No size changes in the non-debug object files

This broke with the changes to the pinned object handling in
67f4ab7e.

The program in #7257 was spending 90% of its time counting the live
data in gen->large_objects.  We already avoid doing this for small
objects, but in this example the old generation was full of large
objects (actually pinned ByteStrings).

Forcing large allocations here can creates serious fragmentation in
some cases, and since the large allocations are only a small
optimisation we should allow the nursery to hoover up small blocks
before allocating large chunks.

This makes compiling DynFlags a lot quicker

This squashes the "out of spill slots" panic that occasionally happens
on x86, by adding instructions to bump and retreat the C stack pointer
as necessary.  The panic has become more common since the new codegen,
because we lump code into larger blocks, and the register allocator
isn't very good at reusing stack slots for spilling (see Note [extra
spill slots]).

I assume that this is what is intended, as it is used with foldl'

and creating one is quite slow

It was always False before