Make the API more Rustful

[zzeroo]

@TheBiggerGuy I create a issue for that topic. I would be very happy if you take this part.

• [x] Returning empty Results over fixed zero ints
• [x] Better converting result values
• [x] Not passing in empty mutable values on get_* methods
• [x] Checking explicitly for 0 result values

First part: https://github.com/zzeroo/libmodbus-rs/pull/7