Add missing Ncalc.Benchmarks project to sln

[Bykiev]

Add missing Ncalc.Benchmarks project to sln

[gumbarros]

Before merging, I think we should study on how to make the Benchmark work with both .csproj and NuGet version. I 100% agree to the benchmark be add the .sln, but if we compile the solution in debug or release and not SignedRelease, it cause errors (specifically at CI)

[Bykiev]

I've made it draft and opened an issue https://github.com/dotnet/BenchmarkDotNet/issues/2574

[Bykiev]

As InProcess toolchain is not supporting benchmarking with NuGet package we can run it on CI when building separate executables

[gumbarros]

Superseded by #220