Don't mutate input array in joinDigits()
Don't mutate input array in joinDigits()

`reverse()` is a nasty API. It returns the reversed array, but also
mutates it in place. Noticed while using `joinDigits()` to log stuff
while developing.

wincent/hextrapolate,cpojer/hextrapolate,cpojer/hextrapolate,wincent/hextrapolate,wincent/hextrapolate,cpojer/hextrapolate