Proposal: Consume iterators instead of slices for Arg methods

[MS-Painter]

Not sure if this is specifically implemented this way on purpose or not,
so this is also a question aside from a proposal -

I personally found it kind of limiting, that for Arg builder methods such as possible_values(),
You need to supply params of type &[&str] .

I was wondering if there would be any issue, for methods such as possible_values(),
To be implemented in a way similar to this:

Alternative implementation might be more useful:

These tweaks compile (didn't fully test out yet if there would be any issue with using them,
since I wasn't sure this is implemented like this on purpose),
and would help out immensely, for similar methods with this specific niche.

If this is something that could be added throughout the codebase, I would be glad to give it a go to contribute 👍

[MS-Painter]

Code block versions of example changes:

pub fn possible_values<I>(mut self, names: I) -> Self where 
    I: Iterator<Item = &'help str> {
        self.possible_vals.extend(names);
        self.takes_value(true)
    }

pub fn possible_values<I, T: 'help>(mut self, names: I) -> Self where 
    I: Iterator<Item = &'help T>,
    T: AsRef<str> {
        self.possible_vals.extend(names.into_iter().map(|name| name.as_ref()));
        self.takes_value(true)
    }

[pksunkara]

Do send a PR. 😄

[MS-Painter]

#2414 I'll try to finish it up soon 👍