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IND_STATIC used to be an error, but at the moment it can happen
as isAlive doesn't look through IND_STATIC as it ignores static
closures. See trac #3956 for a program that hit this error.

The instances (and deriving declarations) have been taken from the ghc-syb
package.

These are no longer used: once upon a time they used to have different
layout from IND and IND_PERM respectively, but that is no longer the
case since we changed the remembered set to be an array of addresses
instead of a linked list of closures.

The list of threads blocked on an MVar is now represented as a list of
separately allocated objects rather than being linked through the TSOs
themselves.  This lets us remove a TSO from the list in O(1) time
rather than O(n) time, by marking the list object.  Removing this
linear component fixes some pathalogical performance cases where many
threads were blocked on an MVar and became unreachable simultaneously
(nofib/smp/threads007), or when sending an asynchronous exception to a
TSO in a long list of thread blocked on an MVar.

MVar performance has actually improved by a few percent as a result of
this change, slightly to my surprise.

This is the final cleanup in the sequence, which let me remove the old
way of waking up threads (unblockOne(), MSG_WAKEUP) in favour of the
new way (tryWakeupThread and MSG_TRY_WAKEUP, which is idempotent).  It
is now the case that only the Capability that owns a TSO may modify
its state (well, almost), and this simplifies various things.  More of
the RTS is based on message-passing between Capabilities now.

Avoids screeds of "event buffer overflowed; event dropped" in
conc059(threaded1).

This fixes #3838, and was made possible by the new BLACKHOLE
infrastructure.  To allow reording of the run queue I had to make it
doubly-linked, which entails some extra trickiness with regard to
GC write barriers and suchlike.

This replaces the global blackhole_queue with a clever scheme that
enables us to queue up blocked threads on the closure that they are
blocked on, while still avoiding atomic instructions in the common
case.

Advantages:

 - gets rid of a locked global data structure and some tricky GC code
   (replacing it with some per-thread data structures and different
   tricky GC code :)

 - wakeups are more prompt: parallel/concurrent performance should
   benefit.  I haven't seen anything dramatic in the parallel
   benchmarks so far, but a couple of threading benchmarks do improve
   a bit.

 - waking up a thread blocked on a blackhole is now O(1) (e.g. if
   it is the target of throwTo).

 - less sharing and better separation of Capabilities: communication
   is done with messages, the data structures are strictly owned by a
   Capability and cannot be modified except by sending messages.

 - this change will utlimately enable us to do more intelligent
   scheduling when threads block on each other.  This is what started
   off the whole thing, but it isn't done yet (#3838).

I'll be documenting all this on the wiki in due course.

I don't think this fixes any bugs as we don't have single-entry thunks
at the moment, but it could cause problems for parallel execution if
we ever did re-introduce update avoidance.

The warning message eliminated is:
> rts/Linker.c:4756:0:
>      warning: nested extern declaration of 'symbolsWithoutUnderscore'

The error messages eliminated are:
> compiler/nativeGen/AsmCodeGen.lhs:875:31:
>     Not in scope: `mkRtsCodeLabel'
> compiler/nativeGen/AsmCodeGen.lhs:879:31:
>     Not in scope: `mkRtsCodeLabel'
> compiler/nativeGen/AsmCodeGen.lhs:883:31:
>     Not in scope: `mkRtsCodeLabel'

The error message eliminated is:
> compiler/nativeGen/AsmCodeGen.lhs:637:16:
>     Not in scope: data constructor `DestBlockId'

This seems to fix this failure:
[...]
** BUILD SUCCEEDED **
rm -f -f GHC-system.pmdoc/*-contents.xml
/Developer/usr/bin/packagemaker -v --doc GHC-system.pmdoc\
             -o /Users/ian/to_release/ghc-6.12.1.20100330/GHC-6.12.1.20100330-i386.pkg
2010-03-31 15:08:15.695 packagemaker[13909:807] Setting to : 0 (null)
2010-03-31 15:08:15.709 packagemaker[13909:807] Setting to : 0 org.haskell.glasgowHaskellCompiler.ghc.pkg
2010-03-31 15:08:15.739 packagemaker[13909:807] relocate: (null) 0
2010-03-31 15:08:15.740 packagemaker[13909:807] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSXMLDocument initWithXMLString:options:error:]: nil argument'
2010-03-31 15:08:15.741 packagemaker[13909:807] Stack: (
    2511962091,
    2447007291,
    2511961547,
    2511961610,
    2432803204,
    453371,
    447720,
    436209,
    435510,
    9986,
    9918
)
make[1]: *** [framework-pkg] Trace/BPT trap
make: *** [framework-pkg] Error 2

Should fix toggling on OS X "Snow Leopard". Diagnosed by Roman Leshchinskiy.