A library adding indexing capabilities to wnfs

The block store is a file based one that stores each block in a file named after the CID under <roo-dir>/blockstore/.

The state needed to re-use the store after a shutdown is made of:

• <roo-dir>/access.key : the access key of the root directory.
• <roo-dir>/forest.cid : the CID of the stored forest.

A simple command line interface is available in examples/cli.rs. Available commands are:

• cargo run --release --example cli -- put <filename> to import a file.
• cargo run --release --example cli -- get <filename> to retrieve a resource and display its default variant as utf-8.
• cargo run --release --example cli -- ls to list the resources imported.
• cargo run --release --example cli -- search <text> to retrieve resources matching .