Fix some corechecker warnings (#470)

* Improve const correctness in string_span

* Improve const correctness in bounds_tests.cpp and byte_tests.cpp

* Improve const correctness in span_tests.cpp

* Improve const correctness in utils_tests.cpp

* Use gsl::owner for dynamically allocated memory in string_span_tests.cpp

* Improve const correctness in string_span_tests.cpp

* Improve const correctness for strided_span_tests.cpp

tests/utils_tests.cpp

@@ -92,7 +92,7 @@ SUITE(utils_tests)
     TEST(narrow)
     {
         int n = 120;
-        char c = narrow<char>(n);
+        const char c = narrow<char>(n);
         CHECK(c == 120);
 
         n = 300;