Move constants to program memory

[kierdavis]

On AVR devices such as the Arduino, any data that is not marked as PROGMEM will be copied into SRAM at startup, as that is the AVR's main address space. This means that a significant amount of SRAM is occupied by data that is never modified and so would be better suited to being stored in the larger, read-only Flash memory segment.

This is easy to do for individual numbers and strings, but gets nontrivial for structs such as CommandHandler as special standard library functions need to be used to read data from flash memory instead of RAM.