Misc. simplifications (esp-rs#298)

* Hide os_adapter

* Remove global is_connected fn

* Extract state management

* Set mode when creating the controller, extract some fns

* Encapsulate WifiMode a bit better

* Simplify setting addresses to static data

* Only set verbose logging once

* Clean up imports

* Deduplicate wifi auth method conversion

* Clean up powersave mode setup

* Remove unnecessary condition

* Explain transmute

* Misc small readability improvements

* Remove unnecessary conversion

* Prepare waiting for multiple interfaces to start and stop

* Pull up wifi interface to caller

* Remove unnecessary binding

* Don't log warnings twice

* Simplify dump_packet

* Explain default config in constructor

* Remove always-panicking new

* Elide lifetime

* Fix sometimes-unused import