Add support for repr structs

[Rexagon]

Currently simple C-like enums are serialized as string enums:

#[derive(Serialize)]
enum Test {
    First = 1,
    Second = 2,
}

type: string
enum:
  - First
  - Second
example: First

It should be something like this:

oneOf:
  - type: integer
    description: First variant
    example: 1
  - type: integer
    description: Second variant
    example: 2