Fix typo in BigInt.addmul_ui.
Add another pidigits version using BigInt this time based on Ledrug version.

Ledrug version is the fastest version currently. It will be interesting to see
how the BigInt version compares.
Reduce default correctness problem size for new pidigits programs.
Merge pull request #560 from thomasvandoren/pidigits-ledrub-with-bigint

Pi digits Ledrug version using BigInt class.

Add another pidigits version using BigInt, this time based on Ledrug version.

Ledrug version is the fastest version currently. It will be interesting to see
how the BigInt version compares.

This also caught a bug in the BigInt.addmul_ui method. It was calling the
incorrect GMP function.