Refactored the C files out into libraries, started work on the BASIC repl (it can sorta do math!)

src/screen.h

@@ -0,0 +1,12 @@
+#ifndef _SCREEN_H_
+#define _SCREEN_H_
+
+extern char _cursor_x;
+extern char _cursor_y;
+
+void setCursorPosition(char x, char y);
+void backupCursor();
+void advanceCursor();
+void blankScreen();
+
+#endif /* _SCREEN_H_ */