Patch for with context

[PokhodenkoSA]

This modifications make jit() decorator use TargetDispatcher from dppl.
Changes made in #57 by @AlexanderKalistratov and @1e-to.

This PR is for discussion about refactoring this modifications into Numba extension.

[reazulhoque]

We want to achieve the following:

@njit
def foo():

# The with context should be enough to dispatch code for the specified target
with dpctl.device_context("opencl:gpu"):
    foo()

[PokhodenkoSA]

Should be eliminated by IntelPython/numba-dpex#271.