py: Add new cstack API for stack checking, with limit margin macro.

Currently the stack limit margin is hard-coded in each port's call to
`mp_stack_set_limit()`, but on threaded ports it's fiddlier and can lead to
bugs (such as incorrect thread stack margin on esp32).

This commit provides a new API to initialise the C Stack in one function
call, with a config macro to set the margin.  Where possible the new call
is inlined to reduce code size in thread-free ports.

Intended replacement for `MP_TASK_STACK_LIMIT_MARGIN` on esp32.

The previous `stackctrl.h` API is still present and unmodified apart from a
deprecation comment.  However it's not available when the
`MICROPY_PREVIEW_VERSION_2` macro is set.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

py/stackctrl.h

@@ -26,8 +26,12 @@
 #ifndef MICROPY_INCLUDED_PY_STACKCTRL_H
 #define MICROPY_INCLUDED_PY_STACKCTRL_H
 
+// This API is deprecated, please use py/cstack.h instead
+
 #include "py/mpconfig.h"
 
+#if !MICROPY_PREVIEW_VERSION_2
+
 void mp_stack_ctrl_init(void);
 void mp_stack_set_top(void *top);
 mp_uint_t mp_stack_usage(void);
@@ -43,6 +47,8 @@ void mp_stack_check(void);
 #define mp_stack_set_limit(limit) (void)(limit)
 #define MP_STACK_CHECK()
 
-#endif
+#endif // MICROPY_STACK_CHECK
+
+#endif // !MICROPY_PREVIEW_VERSION_2
 
 #endif // MICROPY_INCLUDED_PY_STACKCTRL_H