Provide the ability to create a context and command queue from a `cl_context` or `cl_command_queue`

Currently, opencl3 provides the ability to create a `Device` from `cl_device_id`, via the `new` function, but this functionality is not provided for `Context` or `CommandQueue`. Making their constructors public, or allowing passing `&Device` along side a `cl_context` or `cl_command_queue` would both be adequate solutions. 