FlatSharp Support for 64 Bit FlatBuffers

[jamescourtney]

This thread is to document design and other considerations for supporting google/flatbuffers#7537. Support for this in FlatSharp will be nontrivial, but is something that people periodically ask for.

On a whim, the set of issues that need to be resolved are:

• IInputBuffer interface changes from int to ulong for indices (or, nuint).
• Alternative to Span<T> that supports 64 bit indexes. Possibly a SpanWindow<T> that could return a span over a given window.
• Some 64 bit collection replacement for IList<T>, which also only supports 32 bit indexes.

This will constitute a breaking API change for FlatSharp. Further, this will likely require the project to cross a couple of bridges that I've resisted so far:

• Dependencies on non-Microsoft libraries (Microsoft does not provide 64 bit collections, Spans, Memory, Arrays, etc).
• Use of unsafe code

All told, this will be more difficult in C# than it really should be. Further, I'm curious about whether people truly want to see this supported. I'm also curious how we'll cross-validate this with google/FlatBuffers C# given that they are likely to run into many of the same 32 bit limitations that FlatSharp is.

So, the questions are:

• Can FlatSharp (as a C# project) support this?
• What changes and dependencies would be needed to do so? Given this new set of dependencies and changes, should FlatSharp support this?
• Are there vetted, secure, and trustworthy 64 bit implementations of Collections, Memory, Span, and other "primitives" that FlatSharp could leverage?