Use c_str(), not str() for IdString/std::string == and != operators

Use c_str(), not str() for IdString/std::string == and != operators

These operators work by fetching the string from the global string
table and then comparing with the std::string that was passed in as
rhs.

Using str() means that we create a std::string (strlen; malloc;
memcpy), compare for equality (another memcmp if they have the same
length) and then finally free the string.

Using c_str() means that we pass the const char* straight to
std::string's equality operator. This ends up as a call to
std::string::compare (the const char* flavour), which is essentially
strcmp.