Put string literals in flash memory

[jgfoster]

See the discussion of the F() macro here.

[jgfoster]

This may be a bit more complicated since these literals aren't C strings and can't be printed using printf() and friends.

[jgfoster]

According to this,

Classic AVR architecture is referred as modified Harward, because data and memory are in different address spaces (so address 0 could refer to ram or flash, and there are different functions to access them). New AVR (rebranded megaAVR) architecture is instead more conventional as flash and ram share the same address space (like ARM) so there's no need for special treatment if a variable is only in flash (adding const modifier is enough).

Therefore, "F() is useless in megaAVR architecture."

[jgfoster]

It appears that we can recover a substantial amount of memory by using the flash memory for strings. See #249 where we doubled free memory (from 1553 to 3113).

[jgfoster]

Fixed by #251. This gives us over 3000 bytes of free memory.