Add `ForEachByRef` to iterate over arrays by reference to elements

[takanuva15]

The existing lo.ForEach function makes a copy of every element as it iterates, which is problematic for arrays since that prevents them from being modified.

Example:

type Dummy struct {
	f1 string
}

a := []Dummy{{f1: "Hello"}}
fmt.Println(a)
lo.ForEach(a, func(d Dummy, _ int) {
	d.f1 += ", world!"
})
fmt.Println(a)

Output:

[{Hello}]
[{Hello}]

To resolve this, I would like to add a method ForEachByRef which would pass a ref to each array element in the function callback. With the new function, the above code would look like:

a := []Dummy{{f1: "Hello"}}
lo.ForEach(a, func(d *Dummy, _ int) {
	// etc...

If that sounds fine, can I raise a PR to add this function?

[samber]

I wonder if we should create a dedicated sub-package (similar to mutable/ or parallel/), for such methods.

I suppose that a large number of methods might be optimized this way.

[takanuva15]

Hmm yea ForEachByRef is an interesting case because it only applies to Golang slices which have the ability to pull underlying pointers for structs stored in the slice. I also see that the mutable package has a Map function that would allow mutations like ForEachByRef, but it still has the issue that it makes a copy of the struct for every function call which is expensive for big structs.

I think keeping it in the main package makes the most sense, so we could have lo.ForEach, lo.ForEachWhile, and lo.ForEachByRef, just like we have Drop, DropRight, DropWhile etc.

While we're at it, I can call the function ForEachByPtr rather than ForEachByRef since that'd be more clear. How does that sound?