chore: refine feature flags (#1785)

Removes dependencies between Tokio feature flags. For example, `process`
should not depend on `sync` simply because it uses the `mpsc` channel.
Instead, feature flags represent **public** APIs that become available
with the feature enabled. When the feature is not enabled, the
functionality is removed. If another Tokio component requires the
functionality, it is stays as `pub(crate)`.

The threaded scheduler is now exposed under `rt-threaded`. This feature
flag only enables the threaded scheduler and does not include I/O,
networking, or time. Those features must be explictly enabled.

A `full` feature flag is added that enables all features.

`stdin`, `stdout`, `stderr` are exposed under `io-std`.

Macros are used to scope code by feature flag.

tokio/src/process/mod.rs

@@ -116,7 +116,7 @@ mod imp;
 
 mod kill;
 
-use crate::io::{AsyncRead, AsyncReadExt, AsyncWrite};
+use crate::io::{AsyncRead, AsyncWrite};
 use crate::process::kill::Kill;
 
 use std::ffi::OsStr;
@@ -771,7 +771,7 @@ impl Child {
         async fn read_to_end<A: AsyncRead + Unpin>(io: Option<A>) -> io::Result<Vec<u8>> {
             let mut vec = Vec::new();
             if let Some(mut io) = io {
-                AsyncReadExt::read_to_end(&mut io, &mut vec).await?;
+                crate::io::util::read_to_end(&mut io, &mut vec).await?;
             }
             Ok(vec)
         }