* Fix up GPIO macros

* Fix up send string macros

`string` arguments must not be parenthesized

* Fix up miscellaneous macros

* Make indentation uniform (4 spaces)

* Make #ifdef vs #if defined usage consistent

* Reorder standard includes

* Revert indentation changes as per review comments

* Revert #if defined(__AVR__) → #ifdef __AVR__ change

* Change 2 space indent to 4 spaces on a couple of lines

* Replace include guard with #pragma once