Returns a shifted copy of self's digits, value-of-other times.
The second operand must be an integer in the range -precision
through precision. The absolute value of the second operand gives
the number of places to shift. If the second operand is positive,
then the shift is to the left; otherwise the shift is to the right.
Digits shifted into the coefficient are zeros. The sign and
exponent of the first operand are unchanged.