Object variants support

[gabbhack]

Currently, deser does not support deserialization in object variants. I think to implement it is like a serde, but without externally tagged (because it can be done without object variants.) and may be adjacently tagged - https://serde.rs/enum-representations.html

How I see tagged:

type
  MessageEnum = enum
    Request, Response
  Message = object
    id: string
    case kind {.rename("type").}: MessageEnum
    of Request:
      `method`: string
      params: Params
    of Response:
      result: Value

const js = """
{
  "type": "Request",
  "id": "123",
  "method": "321",
  "params": {...}
}
"""

let msg = js.parse().to(Message)
assert msg.kind == Request

How i see untagged:

##[
`untagged` is not for the entire object, because an object can have multiple "cases" with different behaviors.
`deser` tries to match data with each case, the first successfully matched one will be returned, and the `kind` field will be assigned the corresponding value.
]##
type
  MessageEnum = enum
    Request, Response
  Message = object
    id: string
    case kind {.untagged.}: MessageEnum
    of Request:
      `method`: string
      params: Params
    of Response:
      result: Value

const js = """
{
  "id": "123",
  "method": "321",
  "params": {...}
}
"""

let msg = js.parse().to(Message)
assert msg.kind == Request

Blocked by:

• 5 - Right now, deser doesn't know if any field is missing or not. The error handling should help with this. Of course, exception handling should only be generated in the case of untagged.

• nim-lang/Nim/#16504