Implement base64 support in C++

[theoreticalbts]

Currently we use b58, however due to O(n^2) nature of b58 coding, this isn't ideal for large strings.

Let's use base64 instead.

We can't change the default in C++ from base58 to base64 until Golang supports base64. So we should change the C++ and Golang defaults together as part of #169.