Wordlist subclasses 'tuple' so it can be transparently used.
'in' and '.index()' are fast.
Use Wordlist in bip39_is_checksum_valid, which makes that faster.
Some people complained that due to merging the two word lists,
it is difficult to restore from a metal backup, as they planned
to rely on the "4 letter prefixes are unique in bip39 word list" property.
So we colour words that are only in old list.