Add a `multi` helper for reading a slice of args

[octylFractal]

This works similarly to count, but takes &[Args] instead of using a length + cloning the same args.

Name might not be the best -- I avoided using the more obvious map as unlike count this is not analogous to the map attribute.

[Skirmisher]

How does compose (as in object composition) sound? (it may or may not make sense in this context, I'm not sure, but it's what came to mind haha)

[kitlith]

what about for_each?

[csnover]

It looks like I accidentally superseded this PR by not paying attention 😵‍💫 and writing and committing a similar helper function. The differences between the two appear to be:

1. This implementation includes a Vec optimisation, but I don’t think it is needed since Vec takes no arguments anyway so wouldn’t be using this helper;
2. The committed implementation accepts anything that is IntoIterator<Item = Arg>, so it works with slices but also any other thing that is convertible to an iterator;
3. The committed implementation takes the arguments as a parameter into the helper function, rather than indirectly via args, which is hopefully a little less ‘weird’ than needing to pass a slice in with args_raw.

As such, I am going to close this PR for now. Please feel free to reopen if I have made some mistake. Thanks!