Remove VARN, DIR, and some ICOMMAND macros.

[Luis-Licea]

This commit:

• Replaces the last VARN macro in Imprimis (there are no more in Libprimis).
• Replaces ICOMMAND macros in game.cpp with the addicommand function.
• Replaces the DIR macro with the movement function.
• Creates the Direction enum for using within the move_forward and other similar functions.
• Refactors the gameent->sprinting attribute from being an integer into being a bool.