Since I'd like to decouple the new "task-manager-nursery" lowlevel
primitives/abstractions from the higher-level
`TaskManagerNursery`-supporting API(s) and default per-task
supervision-strat and because `._mngr` is already purposed for
higher-level "on-top-of-nursery" patterns as it is.
Deats,
- move `maybe_open_nursery()` into the new mod.
- adjust the pkg-mod's import to the new sub-mod.
- also draft up this idea for an API which stacks `._beg.collapse_eg()`
onto a nursery with the WIP name `open_loose_tn()` but more then
likely i'll just discard this idea bc i think the explicit `@acm`
stacking is more explicit/pythonic/up-front-grokable despite the extra
LoC.
3acf69be8b