Make reraise_kj_exception available to downstream

I'm using Pycapnp in a project, where we compile `.capnp` files directly to
Cython instead of using the dynamic interface (for speed). For this, we need
access to the `reraise_kj_exception` C function defined by Pycapnp. This is not
possible, because Cython does not automatically make this function available to
downstream users.

My previous solution, in #301, was rather flawed. The  file `capabilityHelper.cpp`, where
`reraise_kj_exception` is defined, was bundled into the distribution, so that
this file could be included in downstream libraries. This turns out to be a
terrible idea, because it redefines a bunch of other things like
`ReadPromiseAdapter`. For reasons not entirely clear to me, this leads to
segmentation faults. This PR revers #301.

Instead, in this PR I've made `reraise_kj_exception` a Cython-level function,
that can be used by downstream libraries. The C-level variant has been renamed
to `c_reraise_kj_exception`.

capnp/helpers/capabilityHelper.h

@@ -63,7 +63,7 @@ inline ::kj::Promise<kj::Own<PyRefCounter>> convert_to_pypromise(kj::Promise<voi
     });
 }
 
-void reraise_kj_exception();
+void c_reraise_kj_exception();
 
 void check_py_error();