feat: add a Receivable promise

Author: sbertix

.gitignore

@@ -1,5 +1,7 @@
 .DS_Store
 *.xcodeproj
+.swiftpm/
+.swiftpm/*
 !Followers.xcodeproj
 /.build
 /Packages

.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata

@@ -0,0 +1,40 @@
+//
+//  Receivables+Once.swift
+//  ComposableRequest
+//
+//  Created by Stefano Bertagno on 27/08/21.
+//
+
+import Foundation
+
+public extension Receivables {
+    /// A `struct` defining a `Requester`-based receivable
+    /// receiving immediately either a success or failure.
+    struct Once<Requester: Requests.Requester, Success>: Receivable {
+        /// The underlying result.
+        public let result: Result<Success, Error>
+
+        /// Init.
+        ///
+        /// - parameters:
+        ///     - success: A valid `Success`.
+        ///     - requester: A valid `Requester`.
+        public init(output success: Success, with requester: Requester) {
+            self.result = .success(success)
+        }
+
+        /// Init.
+        ///
+        /// - parameters:
+        ///     - failure: A valid `Error`.
+        ///     - requester: A valid `Requester`.
+        public init(error failure: Error, with requester: Requester) {
+            self.result = .failure(failure)
+        }
+    }
+}
+
+public extension Requester {
+    /// The associated once type.
+    typealias Once<S> = Receivables.Once<Self, S>
+}