Add MemoryExtensions.Count

[bollhals]

Closes #59466

Original implementation from @stephentoub.

Testcases added for single values and the same tests for ReadOnlySpans.
Types tested are byte, int, TInt and string.

Test cases are mostly copied / inspired by other similar methods like Contains.

[dotnet-issue-labeler]

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[dnfadmin]

All CLA requirements met.

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Issue Details

---

Closes #59466

Original implementation from @stephentoub.

Testcases added for single values and the same tests for ReadOnlySpans.
Types tested are byte, int, TInt and string.

Test cases are mostly copied / inspired by other similar methods like Contains.

Author:	bollhals
Assignees:	-
Labels:	area-System.Memory, new-api-needs-documentation
Milestone:	-

[stephentoub]

Thanks for working on this!

[danmoseley]

Perhaps it was already mentioned above, but do you plan to follow up with a PR to add test cases to dotnet/performance?

[stephentoub]

Thanks!