The legacy encoding depends on the lnwire21 version of lnwire, so it will
let us change lnwire after the migration. To make sure it is separated
from the new encoding, we add it to a new package 'legacy'.
We also put common types in a new package 'common', which will house
types that won't change during the migration, and can be used by both
legacy and current serialization code.
