cmd/swarm, swarm/api/client: add HTTP API client and 'swarm ls' command (#3742)

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.

cmd/swarm/main.go

@@ -145,6 +145,15 @@ The output of this command is supposed to be machine-readable.
 			ArgsUsage: " <file>",
 			Description: `
 "upload a file or directory to swarm using the HTTP API and prints the root hash",
+`,
+		},
+		{
+			Action:    list,
+			Name:      "ls",
+			Usage:     "list files and directories contained in a manifest",
+			ArgsUsage: " <manifest> [<prefix>]",
+			Description: `
+Lists files and directories contained in a manifest.
 `,
 		},
 		{