Implement Context.__init__() in cuda.core.experimental._context

[keenan-simpson]

implement the constructor for the Context object

[leofang]

Right now our Context object is really just a non-functional wrapper holding the CUcontext pointer. I'd like to see what @vzhurba01 concludes for green contexts (#112), before committing to full CUDA context support. There might be a chance (best case scenario) that we simply don't need it.