[SMPI] Datatype: Make the dt id a std::string

[SMPI] Datatype: Make the dt id a std::string

Currently, the Datatype::encode() function returns a
const char*. Using a temporary object as returned from
std::to_string() would return a temporary memory address via
.c_str() and hence cannot be used.

Once encode has been moved to std::string, this could potentially
be an int again...