[C#] Optimized serialization path for RepeatedField<T>

[prat0088]

What language does this apply to?

C# proto3

Describe the problem you are trying to solve.

Efficiently serialize thousands to millions of valuetypes (doubles, ints, etc)

Describe the solution you'd like

Improve loop efficiency as in #5828 . That increased performance 2-5x in my case, saving 100s of milliseconds per request. The rest of my handler takes milliseconds, so protobuf serialization accounts for 50-90% of my microservice's response time.

Better yet, on little endian machines, perform a buffer copy for certain value types, although this might be difficult because C# lacks generics/template-specialization like C++.

Describe alternatives you've considered

n/a

Additional context

Work was started here #5828 but this was closed because the submitter was under the impression these optimizations would be coming soon from some work being done on gRPC. A year later this is still outstanding.

It looks like there is a big rewrite of overlapping parts here #5888 that is about ready to be merged. So any work on this should wait until after that request is merged.

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