Convert demo examples for compatibility with CDT

[jakub-zarembinski]

CDT can be switched on by editing the core/config.py file and removing comments in those two lines:

# eosio_cpp_ = ("EOSIO_CPP", ["/usr/local/eosio.cdt/bin/eosio-cpp"])
# eosio_abigen_ = ("EOSIO_ABIGEN", ["/usr/local/eosio.cdt/bin/eosio-abigen"])

An example of annotations required by CDT:

[[eosio::action]]
void hi( account_name user ) {
   logger_info( "debug user name: ", name{user} );
   require_auth( user );
   print( "Hello, ", name{user} );
}

[stefanzarembinski]

Done, will be effective in the next edition.