use alternative methods for base64 encoding/decoding in order to support BoringSSL

[cpapazian]

sxg_codec.c uses BIO_f_base64 for base 64 encoding, but BoringSSL lists this function as deprecated:

// Deprecated functions.
  |  
  | // BIO_f_base64 returns a filter \|BIO\| that base64-encodes data written into
  | // it, and decodes data read from it. \|BIO_gets\| is not supported. Call
  | // \|BIO_flush\| when done writing, to signal that no more data are to be
  | // encoded. The flag \|BIO_FLAGS_BASE64_NO_NL\| may be set to encode all the data
  | // on one line.
  | //
  | // Use \|EVP_EncodeBlock\| and \|EVP_DecodeBase64\| instead.
  | OPENSSL_EXPORT const BIO_METHOD *BIO_f_base64(void);

Any chance we could replace the calls in libsxg with EVP_EncodeBlock/EVP_DecodeBase64 as the comments in BoringSSL suggest?

cc @twifkak @rgs1

[cpapazian]

was able to implement a version of sxg_base64encode_bytes with EVP_EncodeBlock that passes tests:

bool sxg_base64encode_bytes(const uint8_t* src, size_t length,
  size_t ol;
  // note that the length calcualted by EVP_EncodedLength includes the trailing 
  // null byte, but the value returned by EVP_EncodeBlock does not.
  return EVP_EncodedLength(&ol, length) &&
         sxg_buffer_resize(ol - 1, dst) &&
         EVP_EncodeBlock(dst->data, src, length) == ol - 1;
}

issue is that OpenSSL doesn't seem to have EV_EncodedLength and its version of EVP_EncodeBlock returns an int, so if we wanted to have a single implementation for both versions of OpenSSL, we would need to work around the inconsistencies. This is easy enough, since the openssl docs describe the algorithm used by EV_EncodedLength, and length can be cast to the write type for comparison.