Formatter breaks let f: type a. ... = headers when the body spans several lines #9070

The formatter breaks the header of a binding with locally abstract types whenever the body spans several lines, even when the header fits on one line:

let rec eval: type a. expr<a> => a = e =>
  switch e {
  | Int(n) => n
  | Bool(b) => b
  }

is formatted as

let rec eval:
  type a. expr<a> => a =
  e =>
    switch e {
    | Int(n) => n
    | Bool(b) => b
    }

This is the usual way to write a function over a GADT, so every such function gets the three-line header and an extra indentation level. print_value_binding in res_printer.ml puts the header and the body in one group, so a line break in the body breaks every soft line of the header too. The current layout is recorded in tests/syntax_tests/data/printer/expr/expected/newtype.res.txt.

Expected: the header stays on one line when it fits, and the body is laid out as for any other let.