Fast implementations of base 58 encoding and decoding for the XRP ledger.

The new implementations are about 10x faster when encoding and about 15x faster when decoding. The trick is to use base 58^10 as an intermediate step when encoding and decoding. Instead of directly encoding from base 2^256 to base 58, the algorithm first converts from base 2^256 to base 58^10, and then from base58^10 to base 58. Similarly, when decoding, instead of directly decoding from base 58 to base 2^256, it first converts from base 58 to base 58^10, and then from base 58^10 to base 58.

The intermediate base (58^10) was chosen as the largest power of 58 that still fits in a 64-bit unsigned integer. Computing with 64-bit integers instead of 8-bit integers parallelizes many computations.

A gcc extension was use - 128 bit integers. This is used to multiply two 64-bit integers and get the full 128 bit result. This can be done with one Intel instruction, but it is not available in C++ directly. Alternatively, we can inline assembly instructions for the same effect. Visual studio does not have this extension, and will fall back to using the slower reference implementation.

It is compiled in C++-20 mode. This is so the std::span could be used on the interface. However, it would be easy to convert this to C++-17