Adds simple string library

UpsilonTests/StandardLibraryTests/BasicFunctionsTests.cs

@@ -1,6 +1,5 @@
 using System;
 using Upsilon;
-using Upsilon.Evaluator;
 using Xunit;
 
 namespace UpsilonTests.StandardLibraryTests

UpsilonTests/StandardLibraryTests/StringLibraryTests.cs

@@ -0,0 +1,26 @@
+using Upsilon;
+using Xunit;
+
+namespace UpsilonTests.StandardLibraryTests
+{
+    public class StringLibraryTests : TestClass
+    {
+        public StringLibraryTests(StaticScriptFixture fix) : base(fix)
+        {
+        }
+
+        [Fact]
+        public void ByteUse()
+        {
+            var result = Executor.EvaluateScript<long>(@"string.byte(""a"")", Options);
+            Assert.Equal('a', result);
+        }
+
+        [Fact]
+        public void ByteUseIndex()
+        {
+            var result = Executor.EvaluateScript<long>(@"string.byte(""abcd"", 3)", Options);
+            Assert.Equal('c', result);
+        }
+    }
+}