Get task retvals from the nursery after it finishes (aka "fake-gathers considered harmful")

I haven't seen a way to do this in the docs, and it certainly seems very logical/thematic if it were to work like this. The nursery is the starter and finisher of tasks, its sole and entire purpose is to start tasks and ensure they finish in some halfway-sensible way, with some sort of sane control flow.

Therefore it's only logical that the return values of those tasks are collected by the nursery. Return values shouldn't disappear into the void any more than errors. The nursery gathers errors, so it should gather retvals too.

I find myself wanting to gather return values from my tasks somewhat akin to the following, taken from the docs: running a process for an arbitrary amount of time which dynamically spawns an arbitrary number of tasks.

A little bit of searching finds me a recipe to kinda back my way into this functionality faking out a sort of trio.gather, but it seems to me to be a pretty basic primitive of structured concurrency, and if it's common enough to be a recipe, then it's common enough to implement once and get right instead of rewriting it (or at least repasting and in-place-modifying it) every time you call a nursery.

But then I'm quite new to asyncio and structured concurrency. All I know is that I greatly enjoyed NJS's blog post on control flow, and have equally enjoyed my first 100 lines of code. But I would love even more to either 1) have this primitive implemented in trio 2) understand why it's not a primitive in some way. It sure sounds like one: "nurseries spawn tasks, nurseries collect tasks, nurseries call functions, ergo nurseries collect those functions' retvals"

Start with the documentation recipe described in the issue and read the discussion about nursery return values and structured concurrency. Determine whether Trio should expose a nursery-based result-collection primitive and what its semantics should be. Done means the API decision is settled and the behavior is documented and covered by the project’s appropriate checks.