Memory read command support

[froydnj]

I'm not super-happy with hex_value returning Option<u64>, but I think we have to do this to make the remote stub code robust. We should be able to handle the Option-ness in handle_packet -- by returning an error -- so the actual user Handler never knows about the error states.

[luser]

I think that's the right approach--do as much validation/error handling in this crate as possible, and use strong typing to make it easier for users of the crate to do the right thing. Handling malformed requests feels like it should be isolated to this crate, and when we flesh out the Handler trait it should provide only requests with valid data, and the methods should have return types that require valid responses (errors may be valid responses in some cases, certainly).

I also think if possible it'd be great to have default methods on Handler or something like that so that users can get up and running without having to implement a ton of things.