Allow writing the char representation when creating enums
[The postgres protocol](https://www.postgresql.org/docs/10/static/protocol-message-formats.html) (usually) begins every message with a byte which determines what the rest of the message will be. This is a great use-case for a enum. What I'd like to be able to do is this:

```ksy
enums:
  messages:
    '"R"': AuthenticationRequest
    '"K"': BackendKeyData
    '"S"': ParameterStatus
    '"B"': Bind
    '"2"': BindComplete
    '"3"': CloseComplete
    '"C"': CommandComplete' 
    '"d"': CopyData
    '"c"': CopyDone
    '"f"': CopyFail
    '"G"': CopyInResponse
    '"H"': CopyOutResponse
    '"W"': CopyBothResponse  # used for streaming replication
    '"D"': DataRow  # from backend
```

However, this fails:

```
/enums/messages: unable to parse `"W"` as int
```

As a workaround I'm just using a type: 

```ksy
  message:
    seq:
      - id: msg_type
        type: str
        encoding: ASCII
        size: 1
      - id: len
        type: u4
      - id: body
        size: len - 4  # the length includes itself
        type:
          switch-on: msg_type
          cases:
            '"R"': authentication_request
            '"K"': backend_key_data
```

But it'd be nice to not need to.