Implement Go 1.23 iterators

[thiagokokada]

Close issue: #40.

This PR implements Iterator() and ReverseIterator() methods, that allow usage with for ... range in Go 1.23:

for key, value := range m.Iterator() {
    fmt.Println(key, value)
}

for key, value := range m.ReverseIterator() {
    fmt.Println(key, value)
}

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This change is 

[thiagokokada]

I am not sure if I like this name, so I am open for a better name here.

[elliotchance]

Seems fine to me

[thiagokokada]

Go will only run the tests for the current module directory and not for the modules inside subdirectories, so this means that without explicitly running for v2, the tests would run only for github.com/elliotchance/orderedmap, and not for github.com/elliotchance/orderedmap/v2.

[elliotchance]

Thanks for fixing this!

[elliotchance]

Seems fine to me

[elliotchance]

Thanks for fixing this!