Support annotations inside function definitions.

This is currently supported:

[@genType]
let renameABunch = ~pad =>
  [@genType.as "xRenamed"] (~x) => [@genType.as "yRenamed"] (~y) => pad+x+y;

and gets formatted to:

let renameABunch = ~pad =>
  [@genType.as "xRenamed"]
  ((~x) => [@genType.as "yRenamed"] ((~y) => pad + x + y));

Ideally, it could look like this:

[@genType]
let renameABunch = (~pad, [@genType.as "xRenamed"] ~x, [@genType.as "yRenamed"] ~y) => pad + x + y;

For more background: https://github.com/cristianoc/genType/issues/75.

Start by comparing the current and desired formatting examples in this issue, then read the linked genType issue for background on the annotations. Identify the formatter entry point that handles function definitions and verify the change against examples containing annotations on parameters and nested functions; done means the desired form is produced without regressing the currently supported form.