Revisit parsing of attributes, and printing for functions.

[cristianoc]

Context: rescript-lang/syntax#680

• During parsing, attributes outside ( ) would be added to the function, and inside (...) to the pattern. Parsing async adds @res.async to function, and uncurried (. ...) also adds the attribute to the function.
• Printing can be simplified.
• Need checking that this does not break existing code.