[RFC] Device Array and compat Device Vector

[danhoeflinger]

This RFC proposes adding an experimental device_array - a RAII container for USM device memory. It gives users a focused interface for managing device allocations and and provides the most-travelled usage patterns for device_vector.
The RFC also proposed adding a compat namespace and device_vector within it. device_vector wraps device_array and provides a similar set of functionality to Thrust's device_vector.

Four documents:

• README.md - High level decisions, motivation, diagram
• device_array.md - design and API for device_array
• device_vector_compat.md - design and API for device_vector and helper classes
• usage_pattern_study.md - survey of many thrust::device_vector and dpct::device_vector usages, includes analysis of alternatives and why they diverged from Thrust