Since it seems we're building out more and more higher level primitives
in order to support certain parallel style actor trees and messaging
patterns (eg. task broadcast channels), we might as well start a new
sub-package for purely `trio` constructions. We hereby dub this
the realm of `trionics` (like electronics but for trios instead of
electrons).

To kick things off, add an `async_enter_all()` concurrent
exit-stack-like context manager API which will concurrently spawn
a sequence of provided async context managers and deliver their ordered
results but with proper support for `trio` cancellation semantics.
The stdlib's `AsyncExitStack` is not compatible with nurseries not
`trio` tasks (which are cancelled) since as task will be suspended on
the stack after push and does not ever hit a checkpoint until the stack
is closed.

…' will wait on

Trionics improvements from @overclockworked64

Fix type annotations

The api we've made here is actually closer to `asyncio.gather()` but
with opening async context managers instead of funcs. Use another event
to allow for graceful teardown of children on non-cancellation exits
and add a doc string.

Change to `gather_contexts()`, use event for graceful exit

Add a news fragment