Fix tar gz localforage

[rmccrear]

This will allow the ${filename}.zkey*.tar.gz files to be downloaded from a remote server and stored with localforage.

It introduces the pako and js-tar modules to help with uncompressing the files. Then the files are stored as ArrayBuffers with the index key: ${filename}.zkey${a} (where a is a letter [b-k]) to be consumed by the snarkjs module when needed.

[Divide-By-0]

Comment from dm:
Seems to not work for me. I've fixed the bug you got when running (the one about line 145), do you mind trying again with your twitter password reset email?

Also, when you call the wasm you have an extra slash / before the filename, so it doesn't get the wasm correctly (but even fixing that bug leaves it not-working due to the decompressed files).

I've also uploaded .gz files to the bucket per your advice, so we can use those instead if it's easier.

It seems to have this error when reading the zkey files into the buffer:

Uncaught (in promise) Error: Reading out of bounds
at MemFile.readToBuffer (memfile.js:76:1)
at MemFile.read

Edit: lgtm now, thanks for the contribution!