Some fixes, lots of cleaning. Removed Base64.

Fixed some bugs, mostly related to mod function of negative values.

Removed Base64 class. If you need to have compact representation of
BigInt, use Base64 library of your choice, or any similar.

BigInt is now "final", rewritten BigIntMod accordingly.

Recovered old dir structure. I just don't understand why need to have
lot of nested folders that do not coincide with package names.