Implement produce service

internal/produce/README.md

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+# Produce Service
+
+This produce service allows persisting produce in a database. It supports adding multiple produce items, removing a produce item, getting a produce item by produce code, and selecting all produce items from the database.
+
+## Example
+
+```go
+// Create database and table.
+db := ramdb.NewDatabase()
+_ = db.CreateTable("produce", KeyProduceCode)
+
+produceSvc := produce.NewService(db)
+
+produceItem := produce.Item{
+	Code: "A12T-4GH7-QPL9-3N4M",
+	Name: "Lettuce",
+	Price: money.New(346, "USD"),
+}
+
+_ = produceSvc.Add([]Item{produceItem})
+_, _ = produceSvc.Get(produceItem.Code)
+_, _ = produceSvc.All()
+_ = produceSvc.Remove(produceItem)
+```