Fix decompression

[Divide-By-0]

The .tar.gz decompression step doesn't work -- it compresses fine and uploads fine to the s3 bucket (I think), but when downloading and decompressing (see zkp.ts in targz_frontend branch), the files are bigger than when they were uploaded, and the snarkjs fullProve step gives the error 'zkeyb invalid file format'. This details the error.

Solving this will let us add compressed downloads, halving the download size (decompression is very fast). I think we need to use the zlib library in JS to do this: https://nodejs.org/api/zlib.html#zlib_zlib_unzip_buffer_options_callback .

[Divide-By-0]

Done with #28