feat: add Product and ProductBy functions

[JohnDevitt]

Currently playing around with Go on Advent of Code and found that a Product or ProductBy function would be useful, and fit in well with the preexisting Sum and SumBy functions

Used as such:

list := []int{1, 2, 3, 4, 5}
product := lo.Product(list)
// 120

strings := []string{"foo", "bar"}
product := lo.ProductBy(strings, func(item string) int {
    return len(item)
})
// 9

Hope its useful!

[artemnesterenko]

I think "Multiply" could be a better name, because "Product" can be related to Cartesian Product which is also a nice-to-have function

[JohnDevitt]

I think "Multiply" could be a better name, because "Product" can be related to Cartesian Product which is also a nice-to-have function

I'd argue Product is the proper name for the actual functionality here. Multiply in mind in the product of 2 numbers, rather than n numbers. Even in the lodash documentation their multiply function says it "Returns the product" (Possibly a bad naming choice they don't want to introduce a breaking change for)

How would you feel about Product for these functions and a follow-up PR with a CartesianProduct or CrossJoin (I believe this is used in other libraries) function? Both seem a bit clearer to me and better describe their respective operation

[JohnDevitt]

I think "Multiply" could be a better name, because "Product" can be related to Cartesian Product which is also a nice-to-have function

How would you feel about Product for these functions and a follow-up PR with a CartesianProduct or CrossJoin (I believe this is used in other libraries) function?

#567