Syntax for function expressions with attributes is verbose

Currently if I want to get an attribute on an inner function expression I have to break it out, expose an extra arrow, and add some (really unintuitive) parens:

let foo =
  [@attrOnBar]
  (
    (bar) =>
      [@attrOnBaz]
      ((~baz) => [@attrOnQux] ((~qux=?, ()) => {"bar": bar, "baz": baz}))
  );

I would be interested in having a syntax like:

let foo = (
  [@attrOnBar] bar,
  [@attrOnBaz] ~baz,
  [@attrOnQux] ~qux=?,
  (),
) => {"bar": bar, "baz": baz};

This would basically be an extension of the refmting of nested function expressions to a parens style call syntax that also includes attributes - so I think the first version should be valid, but would refmt to the second.

Start with refmt's handling of nested function expressions and attributes, using the two syntax examples in the issue as the behavioral guide. Done means the compact attribute-bearing function-expression syntax is accepted and refmt produces the proposed form while the existing form remains valid.