Implement TryFrom<std::process::Child> for tokio::process::Child

Currently, you can turn a `std::process::Command` into a `tokio::process::Command`, however, if you have a `std::process::Child` there is no way to turn it into a `tokio::process::Child`. This provides a mechanism.

## Context

The use case I'm after is this: I have a small crate for running commands in Rust. I was wondering what a `tokio` feature might look like when I hit a bit of a snag. I can't find a reliable way to convert a `std::process:Child` into something `await`-able. 

The primary interface is this trait https://docs.rs/fun_run/0.6.0/fun_run/trait.CommandWithName.html. It is implemented for `Command` and `&mut Command`. If I want to add a feature flagged `async fn spawn` for `tokio::process::Command`, then I also need a way to reproduce the same interface in my other already-existing implementations. My thought was that for the default implementation, I can produce a regular `std::process::Child` and then, if I can turn it into a `tokio::process::Child`. Then I can standardize on that interface.

Author: schneems

tokio/CHANGELOG.md

@@ -4,6 +4,12 @@ This fixes a regression on the wasm32-unknown-unknown target, where code that
 previously did not panic due to calls to `Instant::now()` started failing. This
 is due to the stabilization of the first time-based metric.
 
+### Added
+
+- process: Implement `TryFrom<std::process::Child>` for `tokio::process::Child` ([#NNNN])
+
+[#NNNN]: https://github.com/tokio-rs/tokio/pull/NNNN
+
 ### Fixed
 
 - Disable time-based metrics on wasm32-unknown-unknown ([#7322])

tokio/src/process/mod.rs

@@ -1212,6 +1212,24 @@ pub struct Child {
     pub stderr: Option<ChildStderr>,
 }
 
+impl TryFrom<StdChild> for Child {
+    type Error = io::Error;
+
+    fn try_from(value: StdChild) -> io::Result<Self> {
+        let spawned_child = imp::build_child(value)?;
+
+        Ok(Child {
+            child: FusedChild::Child(ChildDropGuard {
+                inner: spawned_child.child,
+                kill_on_drop: false,
+            }),
+            stdin: spawned_child.stdin.map(|inner| ChildStdin { inner }),
+            stdout: spawned_child.stdout.map(|inner| ChildStdout { inner }),
+            stderr: spawned_child.stderr.map(|inner| ChildStderr { inner }),
+        })
+    }
+}
+
 impl Child {
     /// Returns the OS-assigned process identifier associated with this child
     /// while it is still running.