Indent long expressions when pretty-printing

[fpoli]

Currently, the pretty-printer doesn't indent long expressions. Indenting them would improve readability.

An example of the current output:

function snap$Binop(self: Ref): Binop$Snap
  requires acc(Binop(self), read())
{
    (unfolding acc(Binop(self), read()) in (self.discriminant == 1 ? (unfolding acc(BinopSub(self.enum_Sub), read()) in (unfolding acc(u32(self.enum_Sub.f$1), read()) in (unfolding acc(u32(self.enum_Sub.f$0), read()) in get$1$Binop$Snap(self.enum_Sub.f$0.val_int, self.enum_Sub.f$1.val_int)))) : (unfolding acc(BinopAdd(self.enum_Add), read()) in (unfolding acc(u32(self.enum_Add.f$1), read()) in (unfolding acc(u32(self.enum_Add.f$0), read()) in get$0$Binop$Snap(self.enum_Add.f$0.val_int, self.enum_Add.f$1.val_int))))))
}

A more readable output:

function snap$Binop(self: Ref): Binop$Snap
  requires acc(Binop(self), read())
{
    (unfolding acc(Binop(self), read()) in
        (self.discriminant == 1 ?
            (unfolding acc(BinopSub(self.enum_Sub), read()) in
                (unfolding acc(u32(self.enum_Sub.f$1), read()) in
                    (unfolding acc(u32(self.enum_Sub.f$0), read()) in
                        get$1$Binop$Snap(self.enum_Sub.f$0.val_int, self.enum_Sub.f$1.val_int)
                    )
                )
            ) :
            (unfolding acc(BinopAdd(self.enum_Add), read()) in
                (unfolding acc(u32(self.enum_Add.f$1), read()) in
                    (unfolding acc(u32(self.enum_Add.f$0), read()) in
                        get$0$Binop$Snap(self.enum_Add.f$0.val_int, self.enum_Add.f$1.val_int)
                    )
                )
            )
        )
    )
}

[marcoeilers]

It's not as fancy as the example shown here, but PR #616 added line breaks and basic indentation.