0.3.4: Enhancements to dpctl device_context

• dpctl is now aware of DPC++ Sycl PI back-ends. The functionality is now exposed via the context interface.
• C API's queue manager was refactored to require back-end.
• Dump functions were enhanced to print back-end information.
• dpct's device_context now requires back-end, device-type, and device-id to be provided in a string format, e.g. opencl:gpu:0.
• dpctl gained support for unint_8 and unsigned long data types.
• Fixed some important bugs found by static analysis.
• Closed issues #51,#33, #53, #73, #95, #83.
• oneAPI Beta 10 tool chain support was added.