Revert "修复 #158，Windows 下 Debug 版本 dll 崩溃问题 (#159)" (#161)

This reverts commit 5ed7ca8.

src/simple_tokenizer.cc

@@ -21,7 +21,7 @@ PinYin *SimpleTokenizer::get_pinyin() {
   return py;
 }
 
-static TokenCategory from_char(unsigned char c) {
+static TokenCategory from_char(char c) {
   if (std::isdigit(c)) {
     return TokenCategory::DIGIT;
   }