Fixes to make BigInt2 compile.

Thought these changes to the constructors would get around the
compiler error. Might be that I just don't get the templating system yet.
Split bigint2 into two other implementations

BigIntPB will try out a packed-binary representation.
BigIntBCD will try out a Binary-Coded-Decimal representation.
Benchmarking each implentation might be overkill.
But if I can get to each one will make for an interesting
comparison against Phobos.

Just a note on BigIntPB. Having trouble getting the Bcd struct
to work so its unit tests are still failing.