Consider putting (slightly longer) struct literals on one line

[hanna-kruppe]

A variant of #418 for proper structs instead of structvariants, inspired by rust-lang/rust#29113 where the diff contains many slight variations of:

-            Formatted { sign: sign, parts: &parts[..1] }
+            Formatted {
+                sign: sign,
+                parts: &parts[..1],
+            }

@marcusklaas assured me that this only happens to "bodies longer than 20 chars", I would prefer if that number was increased (by default - I was told it is configurable). A more complicated heuristic taking the total line length or the number/complexity of the field values into account might also make sense. I can see the benefit of breaking large, complicated literals, but essentially spending four lines of screen real estate on two simple expressions is overkill.

[nrc]

I think the example looks better after reformatting. I think we should have struct lits on one line only in really very simple cases (which is why we have the short heuristic length). I imagine we could use a smarter heuristic here, like double the heuristic width if every value is just an ident or something.