add operators == (attribute_value, std::string)

previously comparisions like c["a"] = "b" were evaluated by 
inline bool operator==(const std::string &a, const t_string &b) 

by casting atribute_value to std::string, and const char * to t_string

we fix this by adding explicit equality operator for these cases.

Not creating t_string also results in significant preformance improvements in some cases.