Optimize calls to std::string::find() and friends for a single char.

tests/pb/test_decoder.cc

@@ -557,7 +557,7 @@ string wrap_text(int32_t fn, const string& text) {
   string wrapped_text = text;
   size_t pos = 0;
   string replace_with = "\n  ";
-  while ((pos = wrapped_text.find("\n", pos)) != string::npos &&
+  while ((pos = wrapped_text.find('\n', pos)) != string::npos &&
          pos != wrapped_text.size() - 1) {
     wrapped_text.replace(pos, 1, replace_with);
     pos += replace_with.size();

upb/bindings/lua/upbc.cc

@@ -19,7 +19,7 @@ class LuaGenerator : public protoc::CodeGenerator {
 };
 
 static std::string StripExtension(absl::string_view fname) {
-  size_t lastdot = fname.find_last_of(".");
+  size_t lastdot = fname.find_last_of('.');
   if (lastdot == std::string::npos) {
     return std::string(fname);
   }

upbc/common.cc

@@ -18,7 +18,7 @@ void AddMessages(const protobuf::Descriptor* message,
 }  // namespace
 
 std::string StripExtension(absl::string_view fname) {
-  size_t lastdot = fname.find_last_of(".");
+  size_t lastdot = fname.find_last_of('.');
   if (lastdot == std::string::npos) {
     return std::string(fname);
   }