Turn the modem on by setting appropriate pins to high/low.
Implemented all commands from
https://github.com/pythings/Drivers/blob/master/SIM800L.py#L116-L143 in Rust.
Moved AP values to a config module. Maybe create a build environment for this
later on, we'll need to support multiple telecom providers for sure ...
Fix the write/read implementation for the Serial. Match on end and handle the
timeout properly for every command.
