Support multiple 'alter column' sub operations

[andrew-farries]

Allow 'alter column' operations to include multiple sub-operations. This means migrations like this one are now possible:

{
  "name": "35_alter_column_multiple",
  "operations": [
    {
      "alter_column": {
        "table": "events",
        "column": "name",
        "name": "event_name",
        "type": "text",
        "nullable": false,
        "unique": {
          "name": "events_event_name_unique"
        },
        "check": {
          "name": "event_name_length",
          "constraint": "length(name) > 3"
        },
        "up": "(SELECT CASE WHEN name IS NULL THEN 'placeholder' ELSE name END)",
        "down": "name"
      }
    }
  ]
}

This 'alter column' operation:

• Renames a column
• Changes its type
• Sets it NOT NULL
• Adds a unique constraint
• Adds a check constraint

Previously, this would have required 5 different operations.

Builds on #337. Fixes #336