Karatsuba algorithm for bigint multiplication

Hi,

I was wondering if a faster algorithm for bigint multiplication could be used for large bigints. The Karatsuba algorithm is relatively simple, so it wouldn't increase the code size by much while being significantly faster. I would love to contribute code for this. (Though I'm not sure what algorithm the project already uses!)

The issue names no files, tests, or entry points. Start by locating JSBI's bigint multiplication implementation and existing tests, then determine which algorithm is currently used and how large operands are benchmarked. Done means an agreed algorithm change supported by correctness tests and performance measurements.

Written by the indexing model from the issue text.