Implement get_backend, get_platform, get_backend_info

[1e-to]

Proposed names:

• DPCTLDevice_GetBackend()
• DPCTLDevice_GetPlatform()
• DPCTLDevice_GetBackendInfo()

[diptorupd]

@1e-to DPCTLDevice_GetBackend() and DPCTLDevice_GetPlatform() are implemented and merged in #300.

[diptorupd]

get_backend_info is out of scope for us for now.