Run_Up should indicate sign change as well as overflow

[simonjwright]

Walker’s comments in Mill.java say for addition

:186 Check for passage through infinity (carry out) and set run up lever if that has occurred. The result is then taken modulo 10^50.

:193 Run up gets set when the result of an addition changes the sign of the first argument. Note that since the same lever is used to indicate carry and change of sign, it is not possible to distinguish overflow from sign change.

and similarly for subtraction at :210, :217.