Implement folder transfer

[piegamesde]

Transferring folders is not that much more complex than transferring files: simply zip all files and send that one with some special metadata. The receiving end might even be able to unzip the thing on the fly, so no temporary file here.

The problem for a streaming send is that we need to know the number of compressed bytes in advance, so that might be a bit tricky but probably nothing insurmountable.

[piegamesde]

Moved to magic-wormhole#96