refactor: move `unixfs` out of the `iroh-resolver` package

[ramfox]

We need to separate out the unixfs definition, unixfs_builder, and hamt work from the rest of the iroh-resolver code.

This is a bit hairy as there is some cross-contamination of dependencies (iroh-resolver::resolver depends on some iroh-resolver::unixfs bits, and vice versa)

[fabricedesre]

That would be great, especially if that is reusable for native ipfs:// and ipns:// protocol handlers in browsers.