Update the chr_at primitive to return characters with the base string's actual encoding.

Over time we've veered away from what Rubinius's `chr_at` primitive does; it may make sense to just rename it. Given the big change in semantics I removed the pointer to the original Rubinius method because it'd just confuse any reader at this point. Attribution still appears in the copyright header. The change in semantics was necessary to get `String#inspect` to properly format strings with dummy encodings.

Author: nirvdrum

src/main/java/org/truffleruby/core/string/StringNodes.java

@@ -3200,42 +3200,43 @@ public Object stringByteSubstring(Object string, Object range, Object length) {
     @ImportStatic(StringGuards.class)
     public static abstract class StringChrAtPrimitiveNode extends CoreMethodArrayArgumentsNode {
 
-        @Child private StringByteSubstringPrimitiveNode stringByteSubstringNode = StringByteSubstringPrimitiveNode.create();
-
         @Specialization(guards = "indexOutOfBounds(string, byteIndex)")
         public Object stringChrAtOutOfBounds(DynamicObject string, int byteIndex) {
             return nil();
         }
 
         @Specialization(guards = { "!indexOutOfBounds(string, byteIndex)", "isSingleByteOptimizable(string, singleByteOptimizableNode)" })
         public Object stringChrAtSingleByte(DynamicObject string, int byteIndex,
+                @Cached("create()") StringByteSubstringPrimitiveNode stringByteSubstringNode,
                 @Cached("create()") RopeNodes.SingleByteOptimizableNode singleByteOptimizableNode) {
             return stringByteSubstringNode.executeStringByteSubstring(string, byteIndex, 1);
         }
 
         @Specialization(guards = { "!indexOutOfBounds(string, byteIndex)", "!isSingleByteOptimizable(string, singleByteOptimizableNode)" })
         public Object stringChrAt(DynamicObject string, int byteIndex,
+                @Cached("create()") EncodingNodes.GetActualEncodingNode getActualEncodingNode,
                 @Cached("create()") RopeNodes.BytesNode bytesNode,
                 @Cached("create()") RopeNodes.CalculateCharacterLengthNode calculateCharacterLengthNode,
                 @Cached("create()") RopeNodes.CodeRangeNode codeRangeNode,
-                @Cached("create()") RopeNodes.SingleByteOptimizableNode singleByteOptimizableNode) {
-            // Taken from Rubinius's Character::create_from.
-
+                @Cached("create()") RopeNodes.SingleByteOptimizableNode singleByteOptimizableNode,
+                @Cached("create()") MakeStringNode makeStringNode) {
             final Rope rope = rope(string);
+            final Encoding encoding = getActualEncodingNode.execute(rope);
             final int end = rope.byteLength();
-            final int c = calculateCharacterLengthNode.characterLength(rope.getEncoding(), codeRangeNode.execute(rope),
-                    bytesNode.execute(rope), byteIndex, end);
+            final byte[] bytes = bytesNode.execute(rope);
+            final int c = calculateCharacterLengthNode.characterLength(encoding, codeRangeNode.execute(rope),
+                    bytes, byteIndex, end);
 
             if (!StringSupport.MBCLEN_CHARFOUND_P(c)) {
                 return nil();
             }
 
-            final int n = StringSupport.MBCLEN_CHARFOUND_LEN(c);
-            if (n + byteIndex > end) {
+            if (c + byteIndex > end) {
                 return nil();
             }
 
-            return stringByteSubstringNode.executeStringByteSubstring(string, byteIndex, n);
+            return makeStringNode.executeMake(
+                    ArrayUtils.extractRange(bytes, byteIndex, byteIndex + c), encoding, CR_UNKNOWN);
         }
 
         protected static boolean indexOutOfBounds(DynamicObject string, int byteIndex) {