Convert to C++ class, clean up code

Convert the entire file into a C++ class (with C wrappers to preserve
the ABI).  This allows for setting individual values of the global
struct(class) in-situ instead of a cryptic list at the end of the struct
definition.  It also removes a lot of redundant `twi.`s from most class
members.

Clean up the code by converting from `#defines` to inline functions, get
rid of ternarys-as-ifs, use real enums, etc.

For slave_receiver.ino, the numbers are:
GIT Master IRAM: 0x723c
This push IRAM: 0x6fc0

For a savings of 636 total IRAM bytes (note, there may be a slight flash
text increase, but we have 1MB of flash to work with and only 32K of IRAM
so the tradeoff makes sense.

Author: earlephilhower