Memoize Calagator::VCalendar#vvenues
Memoize Calagator::VCalendar#vvenues

We call this method once for every event we import, so calculating it
every time is costly (~7min for a 1300-event source on my laptop).

Fixes #481
L2-D2/calagator,L2-D2/calagator,L2-D2/calagator,cp/calagator-1,L2-D2/calagator,cp/calagator-1,reidab/calagator,cp/calagator-1,cp/calagator-1,reidab/calagator,reidab/calagator,reidab/calagator