…code

Refactor the finalisation code of stackless.tasklet and
stackless.channel.
- For tasklets, use a PEP 442 finalizer;
- for channels a simple tp_clear function is sufficient.

Like every PEP 442 other finalizer, the new finalizer runs only once for
each tasklet.

In order to prevent an endless loop in the tasklet-finalizer, try at
most 10 times to kill a tasklet.

Move the handling of tasklets with non-trivial C-state from
tasklet_traverse to tasklet_finalize. If a tasklet still has a
non-trivial C-state after killing the tasklet, the finalizer appends the
tasklet to gc.garbage. Now de-allocation caused by ref-counting behaves
identically to de-allocation caused by GC.
The gc-patch introduced by
stackless-dev/stackless_historic@35ba34f18 is
now obsolete.

Revert the patch, that added the function PyObject_GC_Collectable(). It
was introduced by 
stackless-dev/stackless_historic@35ba34f18.
See bpo-9141 for a discussion of this interesting patch.

Enable soft switching for iterators/coroutines called by "yield from".
Work in progress, not functional, DO NOT MERGE!

…nd coroutines

Improve the finalisation of a soft switched YIELD_FROM instruction.

…ed methods

Enable stackless calls of the following methods:
- generator.send() (generator.__next__() was already stackless);
- coroutine.send();
- coroutine_wrapper.__next__() and coroutine_wrapper.send();
- async_generator_asend.__next__() and async_generator_asend.send().

This change needs more tests.

Fix a error handling in the YIELD_FROM finalisation code.
Add tests and changelog.txt

…t.run"

Enable stackless calls of method "contextvars.Context.run", if
soft-switching is enabled.

…ctions

Better handling of exceptions / NULL result values.
Move the macros to manipulate the try-stackless floag to the API.
Now it is possible to convert existing extension functions to stackless,
without using internal API.
Coming soon: test cases and documentation

Enable stackless calls of coroutines wrapped in "asyncio._CTask", if
soft-switching is enabled.
Needs test cases.
Depends on pull request python#198.

…ctions

Clean up the API header, add tests and documentation.

Remove an unused static function, add test cases and change-log.

instead of _PyRuntime.st.try_stackless.