generate better asts for function bindings

[v-gb]

Currently:

let foo1 : _ = function () -> ()
let foo2 x : _ = function () -> ()

are parsed into these value_bindings:

  (* foo1 *)
  { pvb_args = []
  ; pvb_constraint = Some "_"
  ; pvb_body = Pfunction_cases ...
  }
  (* foo2 *)
  { pvb_args = ["x"]
  ; pvb_constraint = Some "_"
  ; pvb_body = Pfunction_cases ...
  }

I expect instead:

  (* foo1 *)
  { pvb_args = []
  ; pvb_constraint = Some "_"
  ; pvb_body = Pfunction_body (Pexp_function ([], None, Pfunction_cases ...))
  }
  (* foo2 (no changes here) *)
  { pvb_args = ["x"]
  ; pvb_constraint = Some "_"
  ; pvb_body = Pfunction_cases ...
  }

I think the ast for foo1:

• is confusing
• creates a needless distinction between let f : _ = function () -> () vs let f : _ = (function () -> ()), unlike, say, 1 + function () -> () vs 1 + (function () -> ()).
• is essentially an invariant violation. The type of value_bindings in ocamlformat should be understood to be the union of a non-function let-binding + an inline pexp_function node. But the node for foo1 corresponds to the syntax of neither a non-function let-binding (because of body = Pfunction_cases _), nor an inline pexp_function (because pexp_function can't have a type_constraint with an empty list of params).