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);
 }