Improve repr for Union array

[kylebarron]

e.g. this is the repr for a table with a generic geometry column

arro3.core.Table
-----------
id: Utf8
name: Utf8
geometry: Union([(1, Field { name: "", data_type: FixedSizeList(Field { name: "xy", data_type: Float64, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }, 2), nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }), (2, Field { name: "", data_type: List(Field { name: "vertices", data_type: FixedSizeList(Field { name: "xy", data_type: Float64, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }, 2), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }), (3, Field { name: "", data_type: List(Field { name: "rings", data_type: List(Field { name: "vertices", data_type: FixedSizeList(Field { name: "xy", data_type: Float64, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }, 2), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }), (4, Field { name: "", data_type: List(Field { name: "points", data_type: FixedSizeList(Field { name: "xy", data_type: Float64, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }, 2), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }), (5, Field { name: "", data_type: List(Field { name: "linestrings", data_type: List(Field { name: "vertices", data_type: FixedSizeList(Field { name: "xy", data_type: Float64, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }, 2), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }), (6, Field { name: "", data_type: List(Field { name: "polygons", data_type: List(Field { name: "rings", data_type: List(Field { name: "vertices", data_type: FixedSizeList(Field { name: "xy", data_type: Float64, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }, 2), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }), (11, Field { name: "", data_type: FixedSizeList(Field { name: "xyz", data_type: Float64, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }, 3), nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }), (12, Field { name: "", data_type: List(Field { name: "vertices", data_type: FixedSizeList(Field { name: "xyz", data_type: Float64, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }, 3), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }), (13, Field { name: "", data_type: List(Field { name: "rings", data_type: List(Field { name: "vertices", data_type: FixedSizeList(Field { name: "xyz", data_type: Float64, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }, 3), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }), (14, Field { name: "", data_type: List(Field { name: "points", data_type: FixedSizeList(Field { name: "xyz", data_type: Float64, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }, 3), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }), (15, Field { name: "", data_type: List(Field { name: "linestrings", data_type: List(Field { name: "vertices", data_type: FixedSizeList(Field { name: "xyz", data_type: Float64, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }, 3), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }), (16, Field { name: "", data_type: List(Field { name: "polygons", data_type: List(Field { name: "rings", data_type: List(Field { name: "vertices", data_type: FixedSizeList(Field { name: "xyz", data_type: Float64, nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }, 3), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} })], Dense)