Terminate with `float_` function after `#EndFloat`

[cbmarini]

The following two programs now terminate:

#StartFloat 10d
Local F = 3.0;
Local G = 5.0;
Print +s;
.sort

#EndFloat
Local FG = F*G;
Print;
.end

and

#StartFloat 10d
Local F = 3.0;
.sort

#EndFloat
Multiply 5;
Print;
.end

As the float_ functions should become a regular function (but still protected from pattern matching etc.), the correct results should be the product of two float_ functions and 5*float_(...) respectively.

The terminate comes from UnpackFloat. There should probably be a check if the float system is turned on before making the call to UnpackFloat