SafeFileHandle.ThreadPoolValueTaskSource made ligher by one field

[Scooletz]

This PR combines gather/scatter fields in SafeFileHandle.ThreadPoolValueTaskSource into one. This saves one reference field in TPVTS.

[Scooletz]

This is my first PR to the mighty runtime 😅

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[jkotas]

This is a performance optimization. Do you have any benchmark that shows this makes a difference?

[Scooletz]

This is a performance optimization. Do you have any benchmark that shows this makes a difference?

Let me spin some tomorrow and share what it does.

[stephentoub]

@Scooletz, were you able to gather any performance data demonstrating this is a net positive?

While this is shaving off a field, it's on an instance that's typically pooled, so in what's expected to be the common case, it's only saving a field on a single instance of a long-lived object. And in exchange it's paying for a cast on each use. It's not obvious to me this is the right trade-off.

[Scooletz]

I was not able to show anything measurable here. @stephentoub I think we can close it.