Repair some issues with coroutine exception handling

Disable refnanny in PyPy since it otherwise risks to access partly initialised objects, e.g. tuples.

Only reset the frame backlink of *internally* created exception tracebacks in generators/coroutines, not the ones stored away from the caller state.

Also make some C function signatures generic to avoid a dependency on concrete struct types in "far away" code.

This means abs(MIN_INT) has the "overflow" behavior of returning
MIN_INT as in C, but avoids surprising unsigned arithmetic for all
other values.

To preserve Python compatibility, abs(x) dissallows integer inference
for x just like the other potentially overflowing arithmetic operators.

This fixes Github Issue #1837

See discussion at https://github.com/cython/cython/issues/1837

Terminate generator in test to avoid having its exception state hang around, which leads to exception leaks in Py3.3 for the pure Python comparison test.

Check frame back-pointer before we clear it. CPython can simply assert the identity in genobject.c because it always owns a frame itself for the current coroutine. Since Cython does not have frames here and uses the last frame of whatever traceback it currently owns, it's not clear if we can really give any guarantees better than best-effort here.

Move exception state cleanup into generator body code to allow a distinction between normal yields and yields in except blocks. They require different handling according to what CPython does.