QueryStringEnumerable now works in terms of ReadOnlyMemory<char>

[SteveSandersonMS]

While using the new QueryStringEnumerable to implement #33338, I realized it would be far more useful if the APIs were based around ReadOnlyMemory<char> instead of ReadOnlySpan<char>.

This doesn't incur any extra perf cost in realistic cases, because the querystring is basically always going to come from an actual System.String on the heap, so we can represent that as ReadOnlyMemory<char> without any further allocations.

The benefit is that, now:

• It's no longer a ref struct, because it doesn't need to be. Feel free to do normal things with it.
• The actual parsed values (e.g., EncodedName, DecodeName(), etc.) are no longer ref structs either, so you can do things like use them as keys or values in dictionaries, or store them on the heap for any other reason without having to allocate a new string.

I'm not aware of any drawbacks in realistic scenarios.

[ghost]

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[pranavkm]

@halter73 to follow up with the .NET API review folks to figure out if returning ReadOnlyMemory is acceptable. API tentatively approved by ASP.NET Core review.