Plug leaking function_records in cpp_function initialization in case of exceptions (found by Valgrind in #2746) (#2756)

* Plug leaking function_record objects when exceptions are thrown

* Plug leak of strdup'ed strings in function_record

* Some extra comments about the function_record ownership dance

* Clean up the function_record better, in case of exceptions

* Demonstrate some extra function_record leaks

* Change  DeleteStrings template argument to free_strings runtime argument in destruct(function_record *)

* Zero-state unique_function_record deleter object

* Clarify rvalue reference to unique_ptr parameter in initialize_generic

* Use push_back with const char * instead of emplace_back