Merge branch 'master' into gm

Author: tanner0101

Sources/FluentPostgreSQL/PostgreSQLStringModel.swift

@@ -0,0 +1,13 @@
+import Foundation
+
+public protocol PostgreSQLStringModel: Model where Self.Database == PostgreSQLDatabase, Self.ID == String {
+    /// This model's unique identifier.
+    var id: String? { get set }
+}
+
+extension PostgreSQLStringModel {
+    /// See `Model.idKey`
+    public static var idKey: IDKey { return \.id }
+}
+
+public protocol PostgreSQLStringPivot: Pivot, PostgreSQLStringModel { }