Pre-RFC: Function parameter defaults

CAD97: > Our hypothesis is that the remaining cases of optional arguments would almost if not entirely be served just as well by a cheap way to say "use the default value." I like this phrasing Fits nicely, too, with imagining some kind of feature for "well in the same crate you can pass them exactly (to help remember to pass everything in wrapper, for example), but others have to pass the marker that there might be more later, so those additions are not source-semver-breaking". But of course that gets back to "this is 3681 again, isn't it", and how this behaviour fits better in a struct than a function.