Add Count function to slices

[Noah-Wagner]

Adds a Count() function to the slices sub-library.

For an invocation Count(a, b), will return the number of elements in a that are equal to the element b.

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Message from Noah Wagner:

Patch Set 1:

(1 comment)

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Message from Ian Lance Taylor:

Patch Set 1:

(1 comment)

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Message from Gopher Robot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
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https://go.dev/doc/contribute#review for more info and tips to get your
patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
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is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

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Message from Noah Wagner:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Ian Lance Taylor:

Patch Set 1:

(1 comment)

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