Use C#7.3's ReadOnlySpan<byte> optimization for static data

[benaadams]

Better write-up than the roslyn issue https://vcsjones.com/2019/02/01/csharp-readonly-span-bytes-static/

/cc @vcsjones

[BrennanConroy]

@davidfowl Should we do this for SignalR too?

[davidfowl]

@BrennanConroy yep. How many static arrays do we have? It would be nice to autogen them but if we don't have enough then maybe it doesn't matter.

[BrennanConroy]

https://github.com/aspnet/AspNetCore/blob/master/src/SignalR/common/Http.Connections.Common/src/NegotiateProtocol.cs#L17
https://github.com/aspnet/AspNetCore/blob/master/src/SignalR/common/SignalR.Common/src/Protocol/HandshakeProtocol.cs#L21
And we'll have more when we get System.Text.Json protocol https://github.com/aspnet/AspNetCore/blob/master/src/SignalR/common/Protocols.NewtonsoftJson/src/Protocol/NewtonsoftJsonHubProtocol.cs#L24

[benaadams]

Ideally could get the C# compiler to also recognise this pattern:

static ReadOnlySpan<byte> ConnectionIdPropertyNameBytes => Encoding.UTF8.GetBytes("connectionId");

As its pretty impenetrable to cast each char to byte individually

[benaadams]

Added comment dotnet/roslyn#24621 (comment) and raised issue https://github.com/dotnet/csharplang/issues/2212