create higher level base64 functions #377

I was thinking a little about API. My generic proposal is providing a convenient wrapper that would work incrementally. I mean: user provides partial data (like input buffer when reading from file) and output buffer of fixed size. Using for decoding would be something like:

auto decoder = Base64Decoder::new();

std::string input;
input.resize(32 * 1024);

std::string output;
output.resize(16 * 1024);
while (/**/) {
      // read a few kilobytes data from into `input`
  
      const size_t bytes_stored = decoder.decode(input.data(), input.size(), output.data(), output.size());
      // bytes_stored will never be greater than output.size()

      write (output.data(), bytes_stored)

      if input file reached EOF {
          while (decoder.pending_output()) {
              const size_t bytes_stored = decoder.flush(output.data(), output.size());
              write (output.data(), bytes)
         }
      }
}

Of course this flexibility is at cost of performance, but my gut feeling is that if somebody want to process data in chunks, than problem is likely I/O bound.

Another thing for base64 encoding - it would be practical if we allowed wrapping output, for instance:

const size_t max_line_length = 72;
const char* separator = "\n";
encode(input, output, max_line_length, separator);
```

Again, nobody would expect that this variant will be as fast as the plain encoding.

@WojciechMula I'm pinging you later today as I have a major upgrade to the base64 support, with a slightly improved API.

Please see #382 where the base64 API was slightly extended (i.e., we have _safe functions).