gh-140431: Fix GC crash due to partially initialized coroutines (gh-140470)

gh-140431: Fix GC crash due to partially initialized coroutines (gh-140470)

The `make_gen()` function creates and tracks generator/coro objects, but
doesn't fully initialize all the fields. At a minimum, we need to
initialize all the fields that may be accessed by gen_traverse because
the call to `compute_cr_origin()` can trigger a GC.