add benchmark for xamarin

[dpvreony]

Add a benchmark runner for xamarin. will help quantify potential fixes to #421

Colt Bauman 3:11 AM

For anyone interested, last month a PR was merged into BenchmarkDotNet to support running on iOS and Android. Pretty sweet. dotnet/BenchmarkDotNet#1360
Not published to Nuget yet but available on their CI Nuget feed.
jonathanpeppersjonathanpeppers
#1360 Basic BenchmarkDotNet support for Xamarin
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.
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   https://github.com/dotnet/BenchmarkDotNet|dotnet/BenchmarkDotNetdotnet/BenchmarkDotNet | Jan 27th | Added by GitHub
   (526 kB)
   https://user-images.githubusercontent.com/840039/73184210-b105fb80-40e1-11ea-9efb-d29fe4d08c79.png
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