Add Seeds type for preparings seeds for a database

This type is mainly to simplify downstream integrations like
`SharingGRDB`, which already provides such a tool defined directly on
`GRDB.Database`. That tool will be able to simply use this tool,
instead, and future integrations can also leverage it.

Author: stephencelis

Sources/StructuredQueriesCore/Documentation.docc/Articles/InsertStatements.md

@@ -447,3 +447,7 @@ clause:
 ### Statement types
 
 - ``Insert``
+
+### Seeding a database
+
+- ``Seeds``

Sources/StructuredQueriesCore/Seeds.swift

@@ -0,0 +1,105 @@
+/// A type that can prepare statements to seed a database's initial state.
+public struct Seeds: Sequence {
+  let seeds: [any Table]
+
+  /// Prepares a number of batched insert statements to be executed.
+  ///
+  /// ```swift
+  /// Seeds {
+  ///   SyncUp(id: 1, seconds: 60, theme: .appOrange, title: "Design")
+  ///   SyncUp(id: 2, seconds: 60 * 10, theme: .periwinkle, title: "Engineering")
+  ///   SyncUp(id: 3, seconds: 60 * 30, theme: .poppy, title: "Product")
+  ///
+  ///   for name in ["Blob", "Blob Jr", "Blob Sr", "Blob Esq", "Blob III", "Blob I"] {
+  ///     Attendee.Draft(name: name, syncUpID: 1)
+  ///   }
+  ///   for name in ["Blob", "Blob Jr"] {
+  ///     Attendee.Draft(name: name, syncUpID: 2)
+  ///   }
+  ///   for name in ["Blob Sr", "Blob Jr"] {
+  ///     Attendee.Draft(name: name, syncUpID: 3)
+  ///   }
+  /// }
+  /// // INSERT INTO "syncUps"
+  /// //   ("id", "seconds", "theme", "title")
+  /// // VALUES
+  /// //   (1, 60, 'appOrange', 'Design'),
+  /// //   (2, 600, 'periwinkle', 'Engineering'),
+  /// //   (3, 1800, 'poppy', 'Product');
+  /// // INSERT INTO "attendees"
+  /// //   ("id", "name", "syncUpID")
+  /// // VALUES
+  /// //   (NULL, 'Blob', 1),
+  /// //   (NULL, 'Blob Jr', 1),
+  /// //   (NULL, 'Blob Sr', 1),
+  /// //   (NULL, 'Blob Esq', 1),
+  /// //   (NULL, 'Blob III', 1),
+  /// //   (NULL, 'Blob I', 1),
+  /// //   (NULL, 'Blob', 2),
+  /// //   (NULL, 'Blob Jr', 2),
+  /// //   (NULL, 'Blob Sr', 3),
+  /// //   (NULL, 'Blob Jr', 3);
+  /// ```
+  ///
+  /// And then you can iterate over each insert statement and execute it given a database
+  /// connection. For example, using the [SharingGRDB][] driver:
+  ///
+  /// ```swift
+  /// try database.write { db in
+  ///   let seeds = Seeds {
+  ///     // ...
+  ///   }
+  ///   for insert in seeds {
+  ///     try db.execute(insert)
+  ///   }
+  /// }
+  /// ```
+  ///
+  /// > Tip: [SharingGRDB][] extends GRDB's `Database` connection with a `seed` method that can
+  /// > build and insert records in a single step:
+  /// >
+  /// > ```swift
+  /// > try db.seed {
+  /// >   // ...
+  /// > }
+  /// > ```
+  ///
+  /// [SharingGRDB]: https://github.com/pointfreeco/sharing-grdb
+  ///
+  /// - Parameter build: A result builder closure that prepares statements to insert every built row.
+  public init(@InsertValuesBuilder<any Table> _ build: () -> [any Table]) {
+    self.seeds = build()
+  }
+
+  public func makeIterator() -> Iterator {
+    Iterator(seeds: seeds)
+  }
+
+  public struct Iterator: IteratorProtocol {
+    var seeds: [any Table]
+
+    public mutating func next() -> SQLQueryExpression<Void>? {
+      guard let first = seeds.first else { return nil }
+
+      let firstType = type(of: first)
+
+      if let firstType = firstType as? any TableDraft.Type {
+        func insertBatch<T: TableDraft>(_: T.Type) -> SQLQueryExpression<Void> {
+          let batch = Array(seeds.lazy.prefix { $0 is T }.compactMap { $0 as? T })
+          defer { seeds.removeFirst(batch.count) }
+          return SQLQueryExpression(T.PrimaryTable.insert(batch))
+        }
+
+        return insertBatch(firstType)
+      } else {
+        func insertBatch<T: StructuredQueriesCore.Table>(_: T.Type) -> SQLQueryExpression<Void> {
+          let batch = Array(seeds.lazy.prefix { $0 is T }.compactMap { $0 as? T })
+          defer { seeds.removeFirst(batch.count) }
+          return SQLQueryExpression(T.insert(batch))
+        }
+
+        return insertBatch(firstType)
+      }
+    }
+  }
+}