Remove dependency on System.Memory/System.Buffers.

[manfred-brands]

Fixes #4990

[OsirisTerje]

Awesome!

[stevenaw]

LGTM too thanks @manfred-brands !

Incidentally, I also just hit the problem this seeks to fix an hour ago at work so this is a very timely fix for me in that regard.

[manfred-brands]

I looked a but more in the SequenceEquals and the implementation on .NET6+ calls the Span overloads which explains that both have the same performance. Unfortunately the method on .NET Framework is VERY inefficient.

| Method             | Runtime              | Size  | Mean       | Error     | Ratio |
|------------------- |--------------------- |------ |-----------:|----------:|------:|
| Original           | .NET 8.0             | 32768 |   9.413 us | 0.0950 us |  1.00 |
| Vectorized         | .NET 8.0             | 32768 |   1.535 us | 0.0306 us |  0.16 |
| ArraySequenceEqual | .NET 8.0             | 32768 |   1.578 us | 0.0087 us |  0.17 |
|                    |                      |       |            |           |       |
| Original           | .NET Framework 4.6.2 | 32768 |  11.803 us | 0.2333 us |  1.00 |
| Vectorized         | .NET Framework 4.6.2 | 32768 |   1.538 us | 0.0149 us |  0.13 |
| ArraySequenceEqual | .NET Framework 4.6.2 | 32768 | 307.529 us | 1.0167 us | 26.07 |

I therefore updated the code to comment out the call to SequenceEqual for .NET Framework.
This means we either have a dependency on System.Memory/Buffers or for .NET Framework we have the original performance.

I don't know actual usages, but I don't expect StreamsComparer to be used so much.
Also anyone still using .NET Framework knows it is much slower than later versions of .NET for everything.

[manfred-brands]

@nunit/framework-team I am at a loss why nunitlite-runner fails on the .net6.0 build.
It claims it cannot load System.Collections.Immutable version 8.0.0

Both dotnet test and running the test in VS work fine. The correct version of System.Collections.Immutable is in the directory.

I needed to up the version to 8.0.0 for BenchmarkDotnet.
NUnit only uses System.Collections.Immutable for test projects.
We could move the Benchmark project to a separate assembly.
However, I would like to know why it doesn't work.

[manfred-brands]

I added a commit to revert the multi-target running of the benchmarks including changing back to version 6.0.0 of System.Collections.Immutable in order to get a green build.

[OsirisTerje]

I'm not worried about a performance degradation for .net framework. So just let that be imho.

The Immutable 6.0 issue: It only affects the benchmark project, right? If so, if w only run that on net8, I would be fine with that.

[manfred-brands]

The Immutable 6.0 issue: It only affects the benchmark project, right?

No, it is nunit.framework.tests that fails when it determines immutable test cases.

[stevenaw]

I've hit the immutable issue in the tests in the past as well. At the time I had opted to go back to V6 after a bit of debugging without getting anywhere. I'm unsure the root cause myself.

Agreed too about netfx performance. Generally speaking, users on that runtime should already be aware that they aren't getting all the performance they could be