Consolidate stateful C globals under a single struct (bpo-30860)

[akruis]

bpo-30860 consolidates global variables unter the _PyRuntime struct. We should do the same for Stackless globals. See https://bugs.python.org/issue30860 for details.

[akruis]

I just merged pull request #153, but the task is not yet completed.
$ ./python Tools/c-globals/check-c-globals.py still emits a few lines related to Stackless:

  # filename: Stackless/module/channelobject.c
                        NAME                          EXTERNAL   CAPI    SCOPE  
---------------------------------------------------- ---------- ------ ---------
 channel_hook                                                                   
---------------------------------------------------- ---------- ------ ---------

  # filename: Stackless/module/stacklessmodule.c
                        NAME                          EXTERNAL   CAPI    SCOPE  
---------------------------------------------------- ---------- ------ ---------
 slp_error_handler                                                              
---------------------------------------------------- ---------- ------ ---------

  # filename: ~???~
                        NAME                          EXTERNAL   CAPI    SCOPE  
---------------------------------------------------- ---------- ------ ---------
 _Py_CheckRecursionLimit                                 X        X             
 _Py_RefTotal                                            X        X             
 slp_cstack_chain                                        X                      
 slp_enable_softswitch                                   X                      
 slp_initial_tstate                                      X                      
 _slp_schedule_fasthook                                  X                      
 _slp_schedule_hook                                      X                      
 slp_try_stackless                                       X                      
---------------------------------------------------- ---------- ------ ---------

[akruis]

Finally I consolidated all stateful C globals of Stackless into runtime- or interpreter state.

Moved to _PyRuntimeState.st (st is a struct _stackless_runtime_state):

• Globals used during hard C-stack switching in slp_transfer() to _PyRuntimeState.st.transfer
• Global slp_try_stackless to _PyRuntimeState.st.try_stackless
• Static global cstack_cache to _PyRuntimeState.st.cstack_cache
• Static global cstack_cachecount to _PyRuntimeState.st.cstack_cachecount

Moved to struct PyStacklessInterpreterState:

• Global slp_enable_softswitch to field .enable_softswitch.
• Global slp_initial_tstate to field .initial_tstate.
• Static global channel_hook to field .channel_hook.
• Static global slp_error_handler to field .error_handler.
• Global _slp_schedule_fasthook to field .schedule_fasthook.
• Global _slp_schedule_hook to field .schedule_hook.
• Global slp_cstack_chain to field .cstack_chain.
• Static global mem_bomb to field .mem_bomb.
• Static global reduce_frame_func to field .reduce_frame_func.

As a consequence I had to change internal API: the STACKLESS_PROPOSE*-macros to require the thread state as first parameter. (The STACKLESS_PROMOTE*-macros, which are more likely to be used by extension modules, did not change.)