add user data to EnzymeRegisterCallHandler

[martinjrobins]

This adds two items to the C API:

1. a void *data arg to EnzymeRegisterCallHandler so users can supply their own data to the callbacks
2. EnzymeRegisterCallHandler and EnzymeGradientUtilsNewFromOriginal added to the C API header

My particular motivation for 1. is that I need access to the LLVM module pointer in my reverse callback, so the data arg allows me to pass this in. I know the LLVM C++ API has getParent that can be used to get the module from the original call instruction, but I'm using the LLVM C API from rust, which as far as I can see does not have this functionality. In any case, I figure it would be generally useful for users to pass arbitrary data to these callbacks.

[wsmoses]

https://llvm.org/doxygen/IR_2Core_8cpp_source.html#l01940 this should do that

[martinjrobins]

yea, you are right, thanks! Ok I guess I just need 2. then. I'll open another PR with just that