C#: add System.Memory dependency

[jtattermusch]

Also add useful Span-based methods for ByteString

System.Memory dependency is only added for netstandard2.0 target, netstandard1.0 and net45 stay unchanged (we actually could add System.Memory dependency for net45 too, but it seems that most users that will benefit from Span optimizations will already have access to netstandard2.0).

This PR is mostly preparation for future span-based optimizations, but to make at least some use of the new dependency, two utility ByteString methods are being added.

See grpc/grpc-dotnet#30 for more details about the planned serialization/deserialization API.

[jtattermusch]

@jskeet WDYT about changing the target from netstandard1.0 to netstandard1.1?

CC @JunTaoLuo @davidfowl @JamesNK

[jtattermusch]

Looks like this change is mutually exclusive with #5445. I'll reach out to the internal user that requested downgrading LangVersion to see what we can do.

[jskeet]

I think I'd prefer to add a netstandard2.0 target (which comes with a bunch of benefits for 2.0+ users) and only conditionally include System.Memory things.

(Jan, I know a bit more about the LangVersion change - ping me if you need to.)

[JunTaoLuo]

This would be considered a breaking change no? Adding another target for netstandard2.0 (or netstandard 1.1 if you want) would probably be better here.

[JamesNK]

+1

[JunTaoLuo]

Adding a new dependency is also usually considered a "breaking change". Maybe make this conditional on the target framework?

[davidfowl]

Adding a new dependency is also usually considered a "breaking change". Maybe make this conditional on the target framework?

Adding a new dependency isn't a breaking change. Removing one is though. I think we should potentially bump the minor version here.

[jtattermusch]

Minor version will be bumped because this change won't be available before protobuf v3.8.x is out.

[jtattermusch]

I'll wait for #5838 to be merged first.

[jtattermusch]

I added netstandard2.0 target and added conditional dependency on System.Memory for netstandard2.0 (It would have been possible to add also for net45, but I'm not sure how useful that would be).

[JamesNK]

FYI I'm halfway through a large PR to add span-based reader and writer for protobuf. Should be able to create a PR in the next day or two.

[jtattermusch]

@anandolee @jskeet can we review and merge?

[anon17]

I think in the FromStream method you can use the "avoid a copy" branch on netstandard2.0, MemoryStream supports GetBuffer there.