Coro-6.0 - the only real threads in perl

Changes for version 6.0 - 2011-06-29

INCOMPATIBLE CHANGE: unreferenced coro objects will now be destroyed and cleaned up automatically (e.g. async { schedule }).
implement a JIT compiler for part of the thread switch code, which gives a 50% speed improvement on threaded perls, and about 4% on non-thraeded perls (so threaded perls now finally reach about half the speed of non-threaded perls).
slightly modernise Coro::Intro, add section about rouse functions.
avoid DEFSV and ERRSV, giving another 10% improvement in thread switching.
Coro::State->is_destroyed is now called is_zombie.
implement a Coro->safe_cancel method that might fail, but cancels in a "safer" way if it succeeds.
add preliminary support for DEBUGGING perls.
get rid of two hash-accesses when initialising a new Coro - this speeds up coro creation by almost a factor of two.
croak when a coro that is being cancelled tries to block (e.g. while executing a guard block), instead of crashing or deadlocking.
use a more robust and also faster method to identify Coro::State objects - speeds up everything a bit.
implement Coro->cancel in XS for a 20% speed improvement, and to be able to implement mutual cancellation.
speed up context switches by a percent or two by more efficiently allocating context stack entries.
implement Coro->join and Coro->on_destroy in XS for a speedup and a reduction in memory use.
cancelling a coro while it itself is cancelling another coro is now supported and working, instead of triggering an assertion.
be a bit more crash-resistant when calling (buggy) on_destroy callbacks (best effort).
move on_destroy into the slf_frame, to allow extension slf functions to have destructors.
get rid if coro refcounting - simply crash in other interpreter threads by nulling the pointers on clone.
simplify warn/die hook handling when loading Coro - the convoluted logic seems to be no longer neccessary.
use libecb instead of our own home-grown gcc hacks.
document alternatives to Coro::LWP. Please use them :)
work around another mindless idiotic NEEDLESS bug in openbsd/mirbsds sigaltstack. Really. wine suffers from it, erlang suffers from it, and it's known since at least 2006.