Basic BenchmarkDotNet support for Xamarin

[jonathanpeppers]

Fixes: #1343

This is basically an "MVP" of Xamarin support with BenchmarkDotNet.

At a high level:

1. Simple benchmarks work with default config settings.

2. You can write a Xamarin.Forms app that runs benchmarks and displays
   the results. I added a very basic sample of this.

Some of the details to get this working:

• ConsoleExitHandler.CancelKeypress throws
  PlatformNotSupportedException on Xamarin platforms. Just ignore
  this exception.

• Multi-target BenchmarkDotNet.Samples to include netstandard2.0.
  This allows me to reference the benchmarks from the Xamarin.Forms
  shared code. I had to exclude some Windows-specific benchmarks.

• Don't probe for mono binary for InProcess. This was breaking
  InProcess for Xamarin, in general. It's not going to find a mono
  binary that doesn't exist.

• Detect Xamarin.Android / Xamarin.iOS. I used the most common
  Type.GetType lookup to detect each platform: Java.Lang.Object
  and Foundation.NSObject.

• Fix for Directory.CurrentDirectory() as ArtifactPath. The
  current directory on Android is not writeable. Looking for /,
  seemed like a simple fix for Android.

• Default to InProcess for Xamarin platforms. Since we can't find a
  mono binary, we have to run in-process. Xamarin.iOS also shouldn't
  use System.Reflection.Emit until the interpreter is more stable.
  Users of BDN can experiment with the interpreter by configuring
  their benchmarks manually.

This is the first I've looked at this repo, so suggestions welcome! I don't
know this codebase.

The bulk of the new files here is what you get from a new Xamarin.Forms
template in Visual Studio. We can trim that down if needed. Thanks!

[jonathanpeppers]

Questions:

1. The Xamarin projects won't be able to build with dotnet build until we ship .NET 5. Should I just remove them from the main SLN file for now?
2. Is there a doc I should update to mention how to use with Xamarin?

[jonathanpeppers]

1. The Xamarin projects won't be able to build with dotnet build until we ship .NET 5. Should I just remove them from the main SLN file for now?
2. Is there a doc I should update to mention how to use with Xamarin?

@adamsitnik or @AndreyAkinshin any thoughts about this? thanks!

[adambarath]

Hi! Any updates on this? BR

[jonathanpeppers]

@adambarath I never heard anything back on this.

I'll use my best judgement to make this "mergeable" and maybe we'll hear back by then?

[adamsitnik]

Overall the changes introduced to the main part of BenchmarkDotNet look very good to me 👍

It's great that you have added samples, but I would prefer to keep them in a separate solution. It could be called BenchmarkDotNet.Xamarin. My main motivation is to make sure that other contributors don't need to install any new prerequisites to be able to build main solution. We also don't need to introduce any big changes to our CI.

@AndreyAkinshin do you agree on having a standalone solution for the Xamarin purposes?

[adamsitnik]

Looks good to me! Big thanks for adding the docs!

[adamsitnik]

[yauhenipakala]

It looks like the documentation hasn't been deployed yet
https://benchmarkdotnet.org/articles/samples/IntroXamarin.html ?