Simplify representation of class field values and methods

[gpetiot]

Makes #2401 easier.

@Julow some normalizations have been undone (see js_source.ml), but the result is closer, and some indentation has to be fixed, but this tremendously simplifies the formatting, and removes the calls to Sugar.fun_ for Pexp_newtype and for fmt_class_field_kind.

• object.ml.ref can be removed once we fixed the regressions
• in the parser, args_typ_strict_binding can be renamed args_typ_strict_binding (we finally covered every case)

If you have the time to have a look at the indentation that would be very helpful!

[gpetiot]

I tried to split the AST change away from the formatting change, but it runs too deep. I tried to minimize the diff as much as I could instead, and I added a changelog entry for the normalizations and for taking into account the break-colon option.