Add List<T>.CopyTo Span<T> APIs

[felipepessoto]

Add List.CopyTo(Span destination) and List.CopyTo(int sourceIndex, int count, Span destination)

Fix https://github.com/dotnet/corefx/issues/33006

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I watched the Design Review video but I still have some questions (this is my first non-trivial contribution).

1. CopyTo(Span destination) should throw if Span.Length is smaller than List.Count?
2. CopyTo(int sourceIndex, int count, Span destination) should throw if count is greater than destination Span?
3. Do I need to use the ThrowHelper.ThrowXXX methods? Or can I just use throw new XException?
4. Is expected to use Buffer.Memmove? I'm not familiar with this class.

Thanks

@safern @sywhang

[felipepessoto]

@jkotas is it my PR, or CI is broken?

[jkotas]

CI is broken.

[azure-pipelines]

Commenter does not have sufficient privileges for PR 891 in repo dotnet/runtime

[azure-pipelines]

Commenter does not have sufficient privileges for PR 891 in repo dotnet/runtime

[safern]

/azp run runtime-coreclr

[safern]

/azp run runtime-libraries

[carlossanlop]

The new public APIs have been properly documented with triple slash comments. Thanks @felipepessoto. Removing the label.
Adding @safern as reviewer for System.Collections.

[safern]

@carlossanlop, are we still adding this API?

I spoke with @eerhardt and @jkotas and I think we agreed that it makes sense to not add this APIs given we can achieve the same using CollectionsMarshal and also that in order to have a "complete" span support and also, adding these APIs might slow down other operations on List<T> so it seems reasonable to teach people to use CollectionsMarshal instead.

Sorry @felipepessoto for this and thanks for the effort on putting this together, looking forward to your next contribution 😄 -- will close this, if people disagree, we can of course re-open.