Add search methods for packed arrays

[timothyqiu]

Adds the following methods to packed arrays:

• count()
• find()
• rfind()

They are useful because:

• remove_at() requires an index. Currently, only bsearch() provides an index and it requires to sort the array first.
• Array already provides these methods.

[kleonc]

I like adding these methods but I think both find() and rfind() should treat their parameters in the same way. For me treating either [0, n - 1] or [-n, n - 1] as valid index range would be acceptable.

[kleonc]

Also this is not true for the current implementation. For an array with 2 elements -5 (if considered as relative to the end of array) is out of bounds. But with the current implementation it would just search the whole array (and the same would happen for positive values out of bounds).

[timothyqiu]

Yeah, I thought about those too. But this is currently how the corresponding methods in plain Array works.

Do you think I should change the Array implementation & doc in this PR too?

[kleonc]

Do you think I should change the Array implementation & doc in this PR too?

I do think these methods should be changed to behave the same across these classes. But I'd say firstly there should be consensus about what the proper/expected behavior is. If there would be such consensus then sure, I don't have anything against adding such changes in this PR.

On the other hand I guess if it was made like that in the Array then it was somehow accepted? 🤔 So maybe behavior in this PR can remain as is as it's consistent with already existing inconsistency (in the Array). And that whole find()/rfind() inconsistency could (and should) be fixed in another PR.

[timothyqiu]

I created a issue for the current index handling inconsistency among array methods: #60878

Let's fix that in another PR.

[kleonc]

Added docs for rfind() are still incorrect here but I guess they can be also fixed in another PR (the one making find()/rfind() consistent).

[akien-mga]

Thanks!