This PR deprecates the file related RPC calls in favour of an improved HTTP API.
The main aim is to expose a simple to use API which can be consumed by thin
clients (e.g. curl and HTML forms) without the need for complex logic (e.g.
manipulating prefix trie manifests).
This adds a swarm ls command which lists files and directories stored in a
manifest. Rather than listing all files, it uses "directory prefixes" in case there are a
lot of files in a manifest but you just want to traverse it.
This also includes some refactoring to the tests and the introduction of a
swarm/api/client package to make things easier to test.
