4837946: Faster multiplication and exponentiation of large integers
4646474: BigInteger.pow() algorithm slow in 1.4.0
Summary: Implement Karatsuba and 3-way Toom-Cook multiplication as well as exponentiation using Karatsuba and Toom-Cook squaring.
Reviewed-by: alanb, bpb, martin
Contributed-by: Alan Eliasen <eliasen@mindspring.com>