Port System.Memory.Performance.Tests

[adamsitnik]

There is a LOT of Memory benchmarks

• Port
• Make sure we don't measure empty loops:
• Take a short look at CoreCLR Span benchmarks, create new issue for removing duplicated benchmarks

for (int i = 0; i < Benchmark.InnerIterationCount; i++)
{
      Span<char> span = memory.Span;
}

[adamsitnik]

Fixed by #94