Add NativeOverlapped overloads

[qmfrederik]

In line with #419, and also based on my own experience, add overloads which use the System.Threading.NativeOverlapped struct instead of Kernel32+OVERLAPPED. .NET has built-in support for overlapped I/O, so let's use that instead of rolling our own.

In line with the guidance in #487, obsolete these types/overloads instead of removing them.

Closes #419

[qmfrederik]

Hmm, having overloads with both OVERLAPPED* and NativeOverlapped* structs generates conflicts (the code generator would generate two identical overloads with IntPtr parameters), so I removed OVERLAPPED* for now. Happy to revisit if needed.

[AArnott]

This is a binary breaking change. AFAIK we haven't introduced any lately, so I hesitate to take one now unless we're prepared to do a bunch of others as well to minimize the number of releases with breaking changes over time. #288 tracks many ideas for breaking changes.
We could perhaps start a branch with changes like these and merge them all at once, although I'm concerned about merge conflicts too, so I prefer to keep a list of ideas for changes and then we can work on them all at once and merge more quickly.

[qmfrederik]

@AArnott Alternatively, we can add this as a new overload in a .Helper.cs file; but at the moment the code generators would generate an IntPtr-based overload, which would cause conflicts.

[AArnott]

What if we just add conversion operators to our struct so it can be cast to the other one?

[AArnott]

I've heard from the .NET interop folks that each p/invoke function has a fixed cost and they were very happy to hear that we had just one per Win32 function and all our overloads were just thunks into that same one. Rather than creating new p/invokes for the same function, can you add methods to the helper file that simply re-cast the pointer from NativeOverlapped* to OVERLAPPED*? That way we have more performant, less memory hungry code.

[AArnott]

Looks perfect. But we could trim down the xml doc comment weight of it significantly. See my tip.