Remove HashSets over KeyValuePair

From https://github.com/dotnet/runtime/issues/73048:

```
System.Private.CoreLib.dll!System.RuntimeType.GetFieldCandidates(string name, System.Reflection.BindingFlags bindingAttr, bool allowPrefixLookup) Unknown
System.Private.CoreLib.dll!System.RuntimeType.GetFields(System.Reflection.BindingFlags bindingAttr) Unknown
System.Private.CoreLib.dll!System.ValueType.Equals(object obj) Unknown
System.Private.CoreLib.dll!System.Collections.Generic.HashSet>.Remove(System.Collections.Generic.KeyValuePair item) Unknown
```

KeyValuePair doesn't implement GetHashCode/Equals, so we use reflection at runtime.

(I would stay away from records too because by properly ordering the checks one can get a better Equals than what Roslyn comes up with.)