Make refuelling a little more flexible

This removes the link between item size and refuel limit, meaning
working with other items (such as FE items) is a little easier.

src/main/java/dan200/computercraft/shared/turtle/apis/TurtleAPI.java

@@ -303,7 +303,8 @@ public class TurtleAPI implements ILuaAPI
             case 31:
             {
                 // refuel
-                int count = parseCount( args, 0 );
+                int count = optInt( args, 0, Integer.MAX_VALUE );
+                if( count < 0 ) throw new LuaException( "Refuel count " + count + " out of range" );
                 return tryCommand( context, new TurtleRefuelCommand( count ) );
             }
             case 32: