Optional field with default

[anka-213]

Currently, if I want a field to be optional, but have a default instead of Maybe, I would have to do this:

type alias Foo =
  { fieldA : Int
  , newField : Bool
  }

myCodec : Codec Foo
myCodec =
  Codec.object (\fieldA newField -> { fieldA =  fieldA, newField = Maybe.withDefault False newField })
    |> Codec.field "field_a" .fieldA Codec.int
    |> Codec.optionalField "new_field" (Just << .newField) Codec.bool
    |> Codec.buildObject

This is pretty ugly and feels like a hack.
It would be way nicer if it was possible to write something like this (name and argument order is just an example):

myCodec : Codec Foo
myCodec =
  Codec.object Foo
    |> Codec.field "field_a" .fieldA Codec.int
    |> Codec.optionalFieldWithDefault "new_field" .newField Codec.bool False
    |> Codec.buildObject

I think it should be easy to implement, but ObjectCodec being opaque means it can't be implemented externally.
I can submit a PR if this is desired?

[miniBill]

I would definitely merge such PR!

[anka-213]

Nice! Do you have any opinions on name or argument order, or should i just use my own judgment?