Added binding for MurmurHash2

[JohanMabille]

Hi,

This adds cython bindings for MurmurHash2 method. We use the native implementation in xeus and a webasm implementation in jupyterlab. We would need the python bindings for ipykernel.

[honnibal]

Sure, happy to have this. I think you need to add the function though.

The way I have this set up is that the extern is done in the .pyx file, not in the .pxd. This prevents the user code from needing to find the header file. But the consequence is that you won't be able to import the function you just exposed --- it needs to have a Cython function, analogous to the others that are there, and that function needs to be exposed in the .pxd

If the above doesn't make sense to you (I get that Cython's a big language with many mechanisms), I'm happy to make the change.

[JohanMabille]

Ah indeed, sorry for that, I'm new to Cython. I've plugged the MurmurHash2 method into the existing methods to avoid duplicating existing code, and I added a version argument to choose between the different versions. A default value is provided so that it is backward compatible, let me know if you prefer a dedicated bunch of functions for MurmurHash2 instead.

[SylvainCorlay]

Ping @honnibal
It would be super great if this was merged!