refactor: Fix quirks in dataset spec types

[spencerwilson]

• Convert TableSchemaField from an enum w/ one struct-like variant per data type => a struct with fields common to all data types—name/description/constraints—plus a field type_: FieldType. FieldType would hold the data type-specific bits, but currently there are none so the enum is just a "unit-only enum".
• Remove missing_values field, and other unused generated code
• Convert Table.schema from Option<TableSchema> => TableSchema
• Drop remaining relics of typify codegen

If you like you can go commit by commit to see step-by-step how I was able to do all this.

A cousin PR of this one was #98, which has more context on the origin of the cruft removed here.

Test plan

Verified that the serialized representation is safely changed, i.e., it retained the aspects our system actually depends on, while dropping unused bits. Running init in this branch produced a spec like:

{
  "id": "018c1760-7c8e-7ddf-a60c-c97a093adcbe",
  "name": "demo",
  "description": null,
  "version": "0.1.0",
  "accelerated": false,
  "dataset": [
    {
      "name": "GOLDEN_TYPES",
      "description": null,
      "source": {
        "id": "909f4a68-179f-41c0-9ace-d48ba554a33b",
        "path": {
          "type": "snowflake",
          "schema": "TEST_SUITE",
          "table": "GOLDEN_TYPES"
        }
      },
      "schema": {
        "fields": [
          {
            "name": "FLOAT",
            "type": "number",
            "constraints": {
              "required": false
            }
          },
          {
            "name": "TIMESTAMP_TZ",
            "type": "datetime",
            "constraints": {
              "required": false
            }
          },
          {
            "name": "TIMESTAMP_LTZ",
            "type": "datetime",
            "constraints": {
              "required": false
            }
          },
          {
            "name": "VARCHAR",
            "type": "string",
            "constraints": {
              "required": false
            }
          },
          {
            "name": "DATE",
            "type": "date",
            "constraints": {
              "required": false
            }
          },
          {
            "name": "NUMBER",
            "type": "number",
            "constraints": {
              "required": false
            }
          },
          {
            "name": "BOOLEAN",
            "type": "boolean",
            "constraints": {
              "required": false
            }
          },
          {
            "name": "VARIANT",
            "type": "string",
            "constraints": {
              "required": false
            }
          },
          {
            "name": "TIMESTAMP",
            "type": "datetime",
            "constraints": {
              "required": false
            }
          },
          {
            "name": "ARRAY",
            "type": "array",
            "constraints": {
              "required": false
            }
          },
          {
            "name": "TIMESTAMP_NTZ",
            "type": "datetime",
            "constraints": {
              "required": false
            }
          }
        ],
        "primaryKey": [
          "FLOAT"
        ]
      }
    }
  ]
}

[spencerwilson]

Current dependencies on/for this PR:

• main
  • PR refactor: Fix quirks in dataset spec types #207 👈

This stack of pull requests is managed by Graphite.

[ajmasci]

You win the red-diff challenge. 🥇