[WIP] code generation refactor

This PR is based the llvmlite2 branch. My aim was to factor out the code generation machinery from the target context and into a separate facility. CUDA is probably broken as I don't have a device to test (yet).
Using the TargetMachine from EngineBuilder does not work on OSX.  The TargetMachine must  not be configured for "jitdefault" code model.