This could lead to a buffer overrun if the final 0 byte is
written to current_word[j] after the loop.
Also document the limit of passphrase in mnemonic_to_seed.
This makes the pointers to the words constant. It moves 8kb from ram
to flash. It changes the return type of mnemonic_wordlist() to reflect
this change. Everyone calling it should also change the type to
`const char * const *`.