Rename split-loop-with-lambda to slice-with-lambda

The refactoring is now published as Slice on sammancoaching.org:
https://sammancoaching.org/refactorings/slice.html

- git mv ideas/split-loop-with-lambda -> ideas/slice-with-lambda
- rename learning-hour file and update frontmatter name
- update paths in tools/rehearse scripts and report title
- update prose: 'Split Loop with Lambda' -> 'Slice with Lambda'
  in learning hour, exercise READMEs, storyboard, solution comments
- link the published Slice refactoring from the pattern draft

ideas/slice-with-lambda/exercise3/solution/test/BuyersTests.cs

@@ -0,0 +1,20 @@
+using IceCreamBuyers;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using System.Collections.Generic;
+
+namespace IceCreamBuyers.Tests
+{
+    [TestClass]
+    public class BuyersTests
+    {
+        [TestMethod]
+        public void ShoppingListTotalUsesThePricingWePassIn()
+        {
+            // pure pricing function: everything costs 2.
+            // Anke: 2 flavours x 2 scoops x 2 = 8 — deterministic, glanceable.
+            var buyers = new Buyers();
+            var anke = new Buyer("Anke", new List<IceCream> { IceCream.Strawberry, IceCream.Vanilla });
+            Assert.AreEqual(8, buyers.ShoppingListTotal(anke, _ => 2));
+        }
+    }
+}