New Span-based serialization logic (followup for #7351)

[jtattermusch]

Span-based parsing has been added by #7351 , this PR adds the serialization counterpart, which allows serialization to IBufferWriter (which is much more GC friendly).

Based on #7490

Update 2020-06-29:
The new benchmarks (as of commit 4f0afc7) are here: https://gist.github.com/jtattermusch/c9e5389a398f7c79710a9bf12aee215f
For both writing primitives and writing messages the performance is comparable (there are some significant improvement and some slight regressions) to the original state, which seems good enough (as this PR offers an API to serialize directly into a Span and/or to IBufferWriter, which are modern APIs allowing for much better GC efficiency).

[JamesNK]

Is IBufferWriter<byte> supported yet? It is referenced but at the moment all tests write to byte[] or Stream.

[jtattermusch]

Is IBufferWriter<byte> supported yet? It is referenced but at the moment all tests write to byte[] or Stream.

I still need to expose the API and add tests, but vast majority of the internal implementation is already there.

[jtattermusch]

With my few last commits (thanks @JamesNK for the ideas), the writing performance for messages now seems ~at par with the previous state (= no regressions) so I think this PR is ready for final review.
@haberman can you please review?

[haberman]

@gerben-s Can you review please?

[jtattermusch]

The new benchmarks (as of commit 4f0afc7) are here: https://gist.github.com/jtattermusch/c9e5389a398f7c79710a9bf12aee215f
For both writing primitives and writing messages the performance is comparable (there are some significant improvement and some slight regressions) to the original state, which seems good enough (as this PR offers an API to serialize directly into a Span and/or to IBufferWriter, which are modern APIs allowing for much better GC efficiency).

[gerben-s]

Yes this looks good to me

[MaximGurschi]

Is there a timeline, for when the nuget package for C# will be released? Eagerly waiting to be able to use Span with the serialization/deserialization methods you have developed :)

[jtattermusch]

Is there a timeline, for when the nuget package for C# will be released? Eagerly waiting to be able to use Span with the serialization/deserialization methods you have developed :)

If everything goes as planned, it should be available over the course of the next few weeks.