Add a code analyzer to fix certain Span<T> usage

[ahsonkhan]

I wanted to try and create a code analyzer and used the following Span<T> usage scenario as motivation:
Collapse AsSpan().Slice(start, length) into AsSpan(start, length)

This can be used to fix https://github.com/dotnet/corefx/issues/27330 (see dotnet/corefx#29078)

Screenshots:

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Majority of the code here is template code. Focus on the following files:

• samples/SpanUsage/SpanUsage/SpanUsage/SpanUsageAnalyzer.cs
• samples/SpanUsage/SpanUsage/SpanUsage/SpanUsageCodeFixProvider.cs
• samples/SpanUsage/SpanUsage/SpanUsage.Test/SpanUsageUnitTests.cs

TODOs (I will file separate issues, and address outside this PR):

1. Add to corefxlab.sln (and build script to produce NuGet package and VSIX that get uploaded to MyGet) - Add to corefxlab.sln (and build script to produce NuGet package and VSIX that get uploaded to MyGet) #2212
2. Fix VSIX project to run using dotnet cli (via the command line) - Fix VSIX project so it can be built using dotnet cli (via the command line) #2213
   • Currently only builds in VS.
   • D:\GitHub\Fork\corefxlab\samples\SpanUsage\SpanUsage\SpanUsage.Vsix\SpanUsage.Vsix.csproj(94,3): error MSB4019: The imported project "D:\GitHub\Fork\corefxlab\dotnetcli\sdk\2.1.300-preview2-008530\Microsoft\VisualStudio\v15.0\VSSDK\Microsoft.VsSDK.targets" was not found. Confirm that the path in the declaration is correct, and that the file exists on disk.
3. Adjust the Diagnostic location to be more precise (currently highlights the parent tree, which ends up over highlighting whenever there are several method calls chained together). - Consider adjusting the diagnostic location to be more precise #2214
4. Update the VSIX description

Future code analysis rules that we can introduce:

• Memory<T>.Slice(...).Span => Memory<T>.Span.Slice(...) - Extend the SpanUsage code analyzer to check for Memory<T>.Slice(...).Span #2215

cc @dotnet/corefxlab-contrib, @tarekgh, @stephentoub, @atsushikan, @khellang, @VSadov, @KrzysztofCwalina

[ahsonkhan]

We should use this for ASP.NET repos too (SignalR and Kestrel):
https://github.com/aspnet/SignalR/blob/8a3516284e0019dcaaffbe695c88907494382868/src/Common/MemoryBufferWriter.cs#L291
https://github.com/aspnet/KestrelHttpServer/blob/e411cbf7e2910c848221b895cda19b77d7b9761d/benchmarkapps/PlatformBenchmarks/DateHeader.cs#L48
https://github.com/aspnet/SignalR/blob/12b69a0fdbbed797c0f8b6a10d101dc184adde86/test/Microsoft.AspNetCore.SignalR.Tests/EndToEndTests.cs#L110

cc @pakrym, @davidfowl

[ahsonkhan]

Fixed all AsSpan() calls in aspnet:
aspnet/SignalR#1991
aspnet/KestrelHttpServer#2491

[ahsonkhan]

And corefxlab: #2207

[KrzysztofCwalina]

Is it really a sample?

[ahsonkhan]

TODO: 1. Add to corefxlab.sln

It could be considered a sample code analyzer for span. I wanted to start it there, and move it to the solution. Would you be fine with adding it to the solution?

[KrzysztofCwalina]

@terrajobst is working on a package for many .NET analyzers. I think this analyzer might be a good fit for such package.

Having said that, I am totally fine with adding it to this samples project short term.

[ahsonkhan]

@dotnet-bot test Innerloop Ubuntu16.04 Release Build and Test