4.x: ComparisonBenchmarks solution

[akarnokd]

This PR adds a standalone netcoreapp2.1 solution with BenchmarkDotnet and Rx.NET as a NuGet dependency and should enable comparative benchmark runs via changing the dependency on the actual Rx.NET version.

Example comparative benchmark for Range:

BenchmarkDotNet=v0.10.14, OS=Windows 10.0.17134
Intel Core i7-4790 CPU 3.60GHz (Haswell), 1 CPU, 8 logical and 4 physical cores
Frequency=3513588 Hz, Resolution=284.6094 ns, Timer=TSC
.NET Core SDK=2.1.300
  [Host]     : .NET Core 2.1.0 (CoreCLR 4.6.26515.07, CoreFX 4.6.26515.06), 64bit RyuJIT
  DefaultJob : .NET Core 2.1.0 (CoreCLR 4.6.26515.07, CoreFX 4.6.26515.06), 64bit RyuJIT

Rx.NET 3.1.1

Method	N	Mean	Error	StdDev	Gen 0	Allocated
Range	1	1.391 us	0.0059 us	0.0052 us	0.4406	1.81 KB
Range	10	4.460 us	0.0136 us	0.0127 us	1.1368	4.67 KB
Range	100	34.751 us	0.1546 us	0.1446 us	7.7515	31.86 KB
Range	1000	336.790 us	1.0798 us	1.0100 us	73.2422	300.08 KB
Range	10000	3,470.563 us	8.8414 us	8.2702 us	726.5625	2980.11 KB
Range	100000	34,452.518 us	169.7244 us	150.4562 us	7250.0000	29785.81 KB
Range	1000000	341,178.302 us	563.2819 us	439.7734 us	72687.5000	297838.53 KB

Rx.NET 4.0.0

Method	N	Mean	Error	StdDev	Gen 0	Allocated
Range	1	1.115 us	0.0073 us	0.0068 us	0.4082	1.68 KB
Range	10	4.127 us	0.0172 us	0.0152 us	1.1063	4.54 KB
Range	100	34.334 us	0.1415 us	0.1324 us	7.6904	31.73 KB
Range	1000	342.211 us	0.5639 us	0.4998 us	72.7539	299.95 KB
Range	10000	3,411.588 us	10.3184 us	9.6518 us	726.5625	2979.98 KB
Range	100000	33,460.104 us	203.2846 us	190.1526 us	7250.0000	29785.68 KB
Range	1000000	336,870.757 us	772.5311 us	722.6260 us	72687.5000	297838.4 KB

Rx.NET 4.1.0-preview.215

Method	N	Mean	Error	StdDev	Gen 0	Allocated
Range	1	648.9 ns	3.667 ns	3.251 ns	0.1392	584 B
Range	10	2,209.8 ns	7.703 ns	6.828 ns	0.2747	1160 B
Range	100	18,301.3 ns	96.817 ns	90.563 ns	1.6479	6920 B
Range	1000	176,247.7 ns	499.962 ns	467.664 ns	15.3809	64520 B
Range	10000	1,794,827.6 ns	7,230.111 ns	6,763.050 ns	152.3438	640520 B
Range	100000	17,042,315.4 ns	96,952.856 ns	90,689.753 ns	1500.0000	6400520 B
Range	1000000	174,388,878.4 ns	548,979.219 ns	486,655.731 ns	15250.0000	64000520 B

[akarnokd]

This has the drawback that only NuGet/MyGet-released versions can be tested and not the latest development one in the local project. I'd rather go with #731.

[danielcweber]

Check #737. It's based on #730 and merges it with #731. The file structure is in rough shape though, would need cleanup.