Implement `zstd_info()` to extract metadata from compressed data

[coolbutuseless]

zstd_info("file.zst"), zstd_info(raw_vec)

Return a named list:

• compressed_size
• uncompressed_size
• dict_id either character string for the dictionary or NULL
• checksum either the character string xxhash stored in the bitstream or NULL
• NULL if not a compressed object? Or just zstd = FALSE?
• bitstream version?

[coolbutuseless]

Implement is_zstd() for a quick T/F check on whether object/buffer/file/connection contains zstd data.