Apply suggestions from code review

Co-authored-by: Matthew Pope <81593196+popematt@users.noreply.github.com>

Author: austnwil

src/main/java/com/amazon/ion/bytecode/bin11/bytearray/ShortTimestampDecoder.kt

@@ -11,6 +11,8 @@ import java.math.BigDecimal
 
 /**
  * Helper class for decoding the various short timestamp encoding variants from a [ByteArray].
+ *
+ * TODO(perf): avoid auto-boxing the `0` integer for the offset when constructing the Timestamp instance.
  */
 internal object ShortTimestampDecoder {
     private const val MASK_4 = 0b1111

src/test/java/com/amazon/ion/bytecode/bin11/bytearray/PrimitiveDecoderTest.kt

@@ -166,6 +166,8 @@ class PrimitiveDecoderTest {
     @CsvSource(
         " 9223372036854775808, 00000000 00000000 00000000 00000000 00000000 00000000 00000000 10000000 00000000",
         "-9223372036854775809, 11111111 11111111 11111111 11111111 11111111 11111111 11111111 01111111 11111111",
+        "                   1, 00000001 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000",
+        "                  -1, 11111111 11111111 11111111 11111111 11111111 11111111 11111111 11111111 11111111",
     )
     fun testReadFixedIntAsBigInteger(expectedValue: BigInteger, input: String) {
         val data = input.binaryStringToByteArray()

src/test/java/com/amazon/ion/bytecode/bin11/bytearray/ReferenceOpcodeHandlerTest.kt

@@ -32,7 +32,7 @@ internal class ReferenceOpcodeHandlerTest {
             OpCode.VARIABLE_LENGTH_SYMBOL -> ReferenceOpcodeHandler(Instructions.I_SYMBOL_REF)
             OpCode.VARIABLE_LENGTH_BLOB -> ReferenceOpcodeHandler(Instructions.I_BLOB_REF)
             OpCode.VARIABLE_LENGTH_CLOB -> ReferenceOpcodeHandler(Instructions.I_CLOB_REF)
-            else -> fail("Opcode is not a variable-length reference opcode: 0x${opcode.toSingleHexByte()}")
+            else -> fail("Opcode is not a variable-length reference opcode: 0x${opcode.toByte().toHexString()}")
         }
         handler.shouldCompile(input, bytecode)
     }