Basic on-the-fly zstd compression

Prefix every bin-key value with a single uint64 varint containing:
  (3bit store-type)            // 0 for as-is, 1 for basic zstd compression, remainder reserved
    +
  (one reserverd bit)    << 3  // always 0 for now
    +
  (3bit compressability) << 4  // 0 if store-type is 0, otherwise `c := (origLen-compLen) * 8 / origLen`
                               // (using integer math, `origLen > compLen > 0` holds for any non-0 store-type)
    +
  (IPLD block data size) << 7  // 0 if store-type is 0

Include a rudimentary, dictionary-less zstd compressor as the first
non-verbatim storage type 1