Add sandbox command to contain deploy, invoke, read

[leighmcculloch]

What

Add sandbox command to contain the sandbox versions of deploy, invoke, read.

Why

I took a quick look at how a remote deploy command would be added (#41) and it soon became overwhelming thinking about how to have such a command coexist with the code in the sandbox deploy command.

At some point someone had expressed interest in grouping sandbox command under a sandbox super command so as to clearly separate them from commands that invoke remote services. I think it might have been @paulbellamy who had dropped this idea in a doc a long time ago.

We never did it, but I think it will make it easier to implement the following tasks if we have the sandbox variations under sandbox and put the remote variations of the commands under another subcommand such as remote:

• CLI: Deploy a Contract via RPC #41
• CLI: Invoke a contract on network #42
• CLI: Faucet sub-command #142

This change makes no functional changes and is purely moving code into a directory (so it is in a Rust module under sandbox), and inserting the sandbox command in between them.

Close #165

[leighmcculloch]

I didn't include serve in this change because while I assume it'd probably live under sandbox, I didn't want to move the file right now since other people might be working on it.