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

The character literal overload is more efficient. More info at:

http://clang.llvm.org/extra/clang-tidy/checks/performance-faster-string-find.html

modules/core/test/test_misc.cpp

@@ -189,7 +189,7 @@ TEST(Core_OutputArrayCreate, _13772)
 TEST(Core_String, find_last_of__with__empty_string)
 {
     cv::String s;
-    size_t p = s.find_last_of("q", 0);
+    size_t p = s.find_last_of('q', 0);
     // npos is not exported: EXPECT_EQ(cv::String::npos, p);
     EXPECT_EQ(std::string::npos, p);
 }

modules/dnn/src/darknet/darknet_io.cpp

@@ -620,7 +620,7 @@ namespace cv {
                             // read section
                             read_net = false;
                             ++layers_counter;
-                            const size_t layer_type_size = line.find("]") - 1;
+                            const size_t layer_type_size = line.find(']') - 1;
                             CV_Assert(layer_type_size < line.size());
                             std::string layer_type = line.substr(1, layer_type_size);
                             net->layers_cfg[layers_counter]["layer_type"] = layer_type;

modules/dnn/src/tensorflow/tf_importer.cpp

@@ -389,7 +389,7 @@ Pin parsePin(const std::string &name)
 {
     Pin pin(name);
 
-    size_t delimiter_pos = name.find_first_of(":");
+    size_t delimiter_pos = name.find_first_of(':');
     if (delimiter_pos != std::string::npos)
     {
         pin.name = name.substr(0, delimiter_pos);