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.c

@@ -24,7 +24,12 @@
  * THE SOFTWARE.
  */
 
+// This API is deprecated, please use py/cstack.h instead
+
 #include "py/runtime.h"
+
+#if !MICROPY_PREVIEW_VERSION_2
+
 #include "py/stackctrl.h"
 
 void mp_stack_ctrl_init(void) {
@@ -62,3 +67,5 @@ void mp_stack_check(void) {
 }
 
 #endif // MICROPY_STACK_CHECK
+
+#endif // !MICROPY_PREVIEW_VERSION_2