util: implement JoinMap (#4640)

## Motivation

In many cases, it is desirable to spawn a set of tasks associated with
keys, with the ability to cancel them by key. As an example use case for
this sort of thing, see Tower's [`ReadyCache` type][1].

Now that PR #4530 adds a way of cancelling tasks in a
`tokio::task::JoinSet`, we can implement a map-like API based on the
same `IdleNotifiedSet` primitive.

## Solution

This PR adds an implementation of a `JoinMap` type to
`tokio_util::task`, using the `JoinSet` type from `tokio::task`, the
`AbortHandle` type added in #4530, and the new task IDs added in #4630.

Individual tasks can be aborted by key using the `JoinMap::abort`
method, and a set of tasks whose key match a given predicate can be
aborted using `JoinMap::abort_matching`.

When tasks complete, `JoinMap::join_one` returns their associated key
alongside the output from the spawned future, or the key and the
`JoinError` if the task did not complete successfully.

Overall, I think the way this works is pretty straightforward; much of
this PR is just API boilerplate to implement the union of applicable
APIs from `JoinSet` and `HashMap`. Unlike previous iterations on the
`JoinMap` API (e.g. #4538), this version is implemented entirely in
`tokio_util`, using only public APIs from the `tokio` crate. Currently,
the required `tokio` APIs are unstable, but implementing `JoinMap` in
`tokio-util` means we will never have to make stability commitments for
the `JoinMap` API itself.

[1]: https://github.com/tower-rs/tower/blob/master/tower/src/ready_cache/cache.rs

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

.cargo/config

@@ -0,0 +1,2 @@
+# [build]
+# rustflags = ["--cfg", "tokio_unstable"]

README.md

@@ -56,7 +56,7 @@ Make sure you activated the full features of the tokio crate on Cargo.toml:
 
 ```toml
 [dependencies]
-tokio = { version = "1.17.0", features = ["full"] }
+tokio = { version = "1.18.0", features = ["full"] }
 ```
 Then, on your main.rs:
 

tokio-util/Cargo.toml

@@ -29,13 +29,12 @@ codec = ["tracing"]
 time = ["tokio/time","slab"]
 io = []
 io-util = ["io", "tokio/rt", "tokio/io-util"]
-rt = ["tokio/rt", "tokio/sync", "futures-util"]
+rt = ["tokio/rt", "tokio/sync", "futures-util", "hashbrown"]
 
 __docs_rs = ["futures-util"]
 
 [dependencies]
-tokio = { version = "1.6.0", path = "../tokio", features = ["sync"] }
-
+tokio = { version = "1.18.0", path = "../tokio", features = ["sync"] }
 bytes = "1.0.0"
 futures-core = "0.3.0"
 futures-sink = "0.3.0"
@@ -45,6 +44,9 @@ pin-project-lite = "0.2.0"
 slab = { version = "0.4.4", optional = true } # Backs `DelayQueue`
 tracing = { version = "0.1.25", default-features = false, features = ["std"], optional = true }
 
+[target.'cfg(tokio_unstable)'.dependencies]
+hashbrown = { version = "0.12.0", optional = true }
+
 [dev-dependencies]
 tokio = { version = "1.0.0", path = "../tokio", features = ["full"] }
 tokio-test = { version = "0.4.0", path = "../tokio-test" }