Rename files

SRM 584 Div 2 250 p.cpp to Div-2\Easy\SRM 584.cpp
SRM 583 Div 2 900 p.cpp to Div-2\Hard\SRM 583.cpp
SRM 584 Div 2 1000 p.cpp to Div-2\Hard\SRM 584.cpp
SRM 584 Div 2 500 p.cpp to Div-2\Medium\SRM 584.cpp