feat: add stringer implementation for `iter.Pair[T, U]`

[mrchocha]

Please provide a brief description of the change.

Added stringer implementation for iter.Pair[T, U]

Example

pair1 := iter.Pair[string, interface{}]{One: "1", Two:"2"}
fmt.Printf("%s\n", foo)

Output

(1,2)

Which issue does this change relate to? https://github.com/BooleanCat/go-functional/issues/82

Contribution checklist.

• [X] I have read and understood the CONTRIBUTING guidelines
• [X] My code is formatted (make check)
• [X] I have run tests (make test)
• [X] All commits in my PR conform to the commit hygiene section
• [X] I have added relevant tests
• [X] I have not added any dependencies

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (94ac3b8) 100.00% compared to head (2d2a861) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #83 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 24 24 Lines 535 546 +11 ========================================= + Hits 535 546 +11 ``` | [Files Changed](https://app.codecov.io/gh/BooleanCat/go-functional/pull/83?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tom+Godkin) | Coverage Δ | | |---|---|---| | [iter/zip.go](https://app.codecov.io/gh/BooleanCat/go-functional/pull/83?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tom+Godkin#diff-aXRlci96aXAuZ28=) | `100.00% <100.00%> (ø)` | |

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[BooleanCat]

Thanks! 👌