[API Proposal]: `HashCode.Combine(ReadOnlySpan<T>)`

### Background and motivation

Sometimes you need to compute hashcodes for variable amounts of elements or more than 8 of them (current Combine limit).

Having HashCode directly accept spans of values would greatly simplify such code.

BCL example:
https://github.com/dotnet/runtime/blob/c458c40ffc0792b767bc529536388858313eed0a/src/coreclr/System.Private.CoreLib/src/System/Delegate.CoreCLR.cs#L232-L240

### API Proposal

```csharp
namespace System;

public struct HashCode
{
public static int Combine(params ReadOnlySpan values);

public int AddRange(ReadOnlySpan values);
public int AddRange(ReadOnlySpan values, IEqualityComparer? comparer);
}
```

### API Usage

```csharp
public readonly struct EquatableArray(T[] arr)
{
public T[] Values { get; } = arr;
// stuff

public bool Equals(EquatableArray other) => Values.SequenceEquals(other.Values);
public int GetHashCode() => HashCode.Combine(Values);
}

Dictionary, int> dict = new();
dict.Add(new([1, 2, 3]), 4);
```

### Alternative Designs

Use an instance and loop with adds.

### Risks

_No response_

Start by comparing the proposed HashCode APIs with the existing Combine and Add patterns, then read the referenced Delegate.CoreCLR.cs example. Check how variable-length spans and optional comparers should behave, including the EquatableArray usage shown. Done means the API design is resolved and supports the stated variable-element hashing scenario.

Written by the indexing model from the issue text.