Improve the benchmark

Compared phi.BigInt to std.BigInt for small and large numbers.
Looks like the standard library type is much faster.

The numbers vary a bit. But here are the results of a sample run.

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Average execution time of built-in long addition = 142.061 ns
Average execution time of phi.BigInt addition for small numbers = 953.79 ns
Average execution time of std.BigInt addition for small numbers = 347.767 ns
Average execution time of phi.BigInt addition for large numbers = 296899 ns
Average execution time of std.BigInt addition for large numbers = 16295.6 ns