chg: update tasks.lib

Improve handling of internal data structures, especially intvecs.
add: provide fetch and imap for Z/p(a) <--> Q(a)
Merge pull request #482 from steenpass/tasks

chg: update tasks.lib
Merge pull request #484 from steenpass/fetch_ext

add: provide fetch and imap for Z/p(a) <--> Q(a)