libgolang/gevent: Put explicit try/catch boundary for tasks spawned via go

Else as https://github.com/python-greenlet/greenlet/pull/285
demonstrates there can be segmentation faults and crashes due to
exceptions from one greenlet propagating to C stack of another greenlet.

No test here. I've tried to do it, but with gevent (contrary to plain
greenlets), spawning new task only schedules corresponding greenlet to
run in the end of current event loop cycle instead of switching to
created greenlet immediately. With this delaying, it was hard for me to
develop corresponding test in a reasonable time.

Hopefully having the test I've done for greenlet itself + hereby
protection is good enough.

/reviewed-on nexedi/pygolang!17

golang/runtime/_runtime_gevent.pyx

 # cython: language_level=2
-# Copyright (C) 2019-2020  Nexedi SA and Contributors.
+# Copyright (C) 2019-2022  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -48,12 +48,17 @@ from golang.runtime._libgolang cimport _libgolang_runtime_ops, _libgolang_sema,
         STACK_DEAD_WHILE_PARKED, panic
 from golang.runtime cimport _runtime_thread
 from golang.runtime._runtime_pymisc cimport PyExc, pyexc_fetch, pyexc_restore
+from golang cimport topyexc
 
 
 # _goviapy & _togo serve go
 def _goviapy(_togo _ not None):
     with nogil:
-        _.f(_.arg)
+        # run _.f in try/catch to workaround https://github.com/python-greenlet/greenlet/pull/285
+        __goviapy(_.f, _.arg)
+cdef nogil:
+    void __goviapy(void (*f)(void *) nogil, void *arg) except +topyexc:
+        f(arg)
 
 @final
 cdef class _togo: