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/includes/capnp_cpp.pxd

@@ -5,7 +5,7 @@ cdef extern from "capnp/helpers/checkCompiler.h":
 
 from libcpp cimport bool
 from capnp.helpers.non_circular cimport (
-    reraise_kj_exception, PyRefCounter,
+    c_reraise_kj_exception as reraise_kj_exception, PyRefCounter,
 )
 from capnp.includes.schema_cpp cimport (
     Node, Data, StructNode, EnumNode, InterfaceNode, MessageBuilder, MessageReader, ReaderOptions,

capnp/includes/schema_cpp.pxd

@@ -2,7 +2,7 @@
 # distutils: language = c++
 
 from libc.stdint cimport *
-from capnp.helpers.non_circular cimport reraise_kj_exception
+from capnp.helpers.non_circular cimport c_reraise_kj_exception as reraise_kj_exception
 
 from capnp.includes.types cimport *