Fix lowercase unicode escaped sequence (#42)

Author: fabianfett

Sources/PureSwiftJSONParsing/DocumentReader.swift

@@ -219,7 +219,11 @@ public struct DocumentReader {
     case 48...57:
       return ascii - 48
     case 65...70:
+      // uppercase letters
       return ascii - 55
+    case 97...102:
+      // lowercase letters
+      return ascii - 87
     default:
       return nil
     }

Tests/JSONParsingTests/StringParserTests.swift

@@ -28,6 +28,12 @@ class StringParserTests: XCTestCase {
     XCTAssertNoThrow(result = try JSONParser().parse(bytes: [UInt8](#""\u005A""#.utf8)))
     XCTAssertEqual(result, .string("Z"))
   }
+    
+  func testSimpleLowercaseEscapedUnicode() {
+    var result: JSONValue?
+    XCTAssertNoThrow(result = try JSONParser().parse(bytes: [UInt8](#""\u003c""#.utf8)))
+    XCTAssertEqual(result, .string("<"))
+  }
 
   func test12CharacterSequenceUnicode() {
     // from: https://en.wikipedia.org/wiki/UTF-16#Examples