impl Source for IntoIterator and slices instead of Vec

[ghost]

I find the impl Source for Vec in the current version of the library ugly. It requires the list of config sources to always be a Vec, which is ugly and constraining.

I think it would be a much better idea to instead impl Source for slices and for IntoIterator.

impling it for slices would allow using static arrays as well as Vecs.

impling it for IntoIterator would make it possible to use the merge method on any Iterator of sources, without collecting into a Vec. This makes things more elegant and removes unnecessary performance overhead when working with things like glob (such as the glob example in the repo).

I think this change would make the API more elegant and would be desirable for version 1.0.

[mehcode]

I agree. Thanks for the suggestion.