Co-authored-by: Tom Prince <tom.prince@private.storage>

avoids ambiguities and implementation problems of iso8601 duration strings

go back to aniso8601 dependency, since we no longer get anything distinctive
from isodate

We're going to start looking at it.  It's not reasonable for its value to be None.

The redeemer will submit this many tokens with its redemption request now.  It
needs to agree with whatever the issuer wants to do, of course.