Add CxxString::clear method
diff --git a/src/cxx.cc b/src/cxx.cc
index 4080ad6..f22e88c 100644
--- a/src/cxx.cc
+++ b/src/cxx.cc
@@ -22,6 +22,8 @@
   return s.length();
 }
 
+void cxxbridge1$cxx_string$clear(std::string &s) noexcept { s.clear(); }
+
 void cxxbridge1$cxx_string$push(std::string &s, const std::uint8_t *ptr,
                                 std::size_t len) noexcept {
   s.append(reinterpret_cast<const char *>(ptr), len);