Add USM allocations

[szymon-zadworny]

This PR provides:

• host, shared and device-side USM allocator bindings in accordance with the upcoming Allocator trait
• a basic Buffer abstraction
• unsafe Buffer allocation functions.

It serves as plumbing work for further safe abstractions. In particular - we can't implement safe allocations without having access to memcpy/fill first, which in turn require sycl::event bindings.