[#89] Add JsonName attribute for union cases with int or bool tags

[Tarmil]

• [x] Create JsonNameAttribute which can be constructed with a string, an int or a bool.
• [x] Support JsonNameAttribute on union cases to enable int or bool tags.

  type MyUnion =
      | [<JsonName 1>] One of name: string
      | [<JsonName 2>] Two of string
  // eg: One "test" --> {"Case":1,"Fields":{"name":"test"}}

  They are stringified when used as JSON property names (eg with ExternalTag: {"1":["test"]}).

• [x] Support JsonNameAttribute on record properties.
• [x] Support multiple JsonNameAttributes on the same item. The first one is used for serialization, and all can be used for deserialization.

  type MyUnion =
      | [<JsonName 1; JsonName "a">] One of string
      | [<JsonName 2>] Two of string

• [x] Support JsonNameAttribute on union cases to specify the name of a union field.

  type MyUnion =
      | [<JsonName("name", "theName")>] One of name: string
  // eg: One "test" --> {"Case":"One","Fields":{"theName":"test"}}

[bartelink]

The first one is used for serialization

I dont believe order is guaranteed to be preserved when you load?

Support JsonNameAttribute on union cases to specify the name of a union field.

Is this in addition to using it directly on the field? Or as a workaround for that not being possible? (My instinct is that this is a pretty baroque design even though it will help source legibility)

[Tarmil]

I dont believe order is guaranteed to be preserved when you load?

Well spotted! I think we can make JsonName take a [<ParamArray>] otherNames: obj[] instead of using multiple attributes. This way the order is guaranteed.

type MyUnion =
    | [<JsonName(1, "a")>] One of string
    | [<JsonName 2>] Two of string

An alternative would be to add an optional parameter eg isMain: bool, but that feels too verbose.

Is this in addition to using it directly on the field? Or as a workaround for that not being possible? (My instinct is that this is a pretty baroque design even though it will help source legibility)

Unfortunately there is no syntax in F# to put an attribute directly on a union field. The following doesn't compile:

type MyUnion =
    | One of [<JsonName "theName">] name: string

So my proposal above is the only way I can think of to support using JsonName for a union field.

[Tarmil]

I realized that we can't have both an optional argument for the union field name and a ParamArray for alternate names. I think I'll make the union field name a mutable property instead, so that the syntax would be:

type MyUnion =
    | [<JsonName 1>]
      [<JsonName("theName", "theOtherName", Field = "name")>]
      One of name: string

// eg: One "value" --> {"Case":1,"Fields":{"theName":"value"}}
//                  or {"Case":1,"Fields":{"theOtherName":"value"}}