GH-3: Refactor to allow to state tracking of sockets, take an &mut SPI while 'activated'

The activated mode allows one to call functions without passing the SPI reference
each time, while not consuming it for the lifetime of W5500.

Cargo.toml

 [package]
 name = "w5500"
-version = "0.1.6"
+version = "0.2.0"
 authors = ["Michael Watzko <michael@watzko.de>"]
 repository = "https://github.com/kellerkindt/w5500.git"
 description = "W5500 IoT Controller implementation. Currently UDP sending and receiving is working. WIP"