Added field information to Union types

[dkcumming]

This PR adds the field types of a union to the JSON. A more thorough investigation into unions should be done, but this is needed for iterator code with MaybeUninit present

Details

A union like:

union Union {
    signed: i8,
    unsigned: u8,
}

currently serialises to:

[
  27,
  {
    "UnionType": {
      "name": "Union",
      "adt_def": 7,
      "layout": {
        "fields": {
          "Union": 2
        },
        "variants": {
          "Single": {
            "index": 0
          }
        },
        "abi": {
          "Aggregate": {
            "sized": true
          }
        },
        "abi_align": 1,
        "size": {
          "num_bits": 8
        }
      }
    }
  }
]

and afterwards serialises to:

[
  27,
  {
    "UnionType": {
      "name": "Union",
      "adt_def": 7,
      "fields": [
        2,
        9
      ],
      "layout": {
        "fields": {
          "Union": 2
        },
        "variants": {
          "Single": {
            "index": 0
          }
        },
        "abi": {
          "Aggregate": {
            "sized": true
          }
        },
        "abi_align": 1,
        "size": {
          "num_bits": 8
        }
      }
    }
  }
]

where 2 is i8, and 9 is u8.