std: reorganize pipe implementations
Currently, there are two distinct types called `AnonPipe` in `std`:
* The one used to implement `io::pipe` (in `sys::anonymous_pipe`)
* The one used to implement `Stdin`/`Stdout`/`Stderr` (in `sys::pal::pipe`)
On Windows, these actually have different semantics, as one of the handles returned by the `sys::pal::pipe` version is opened for asynchronous operation in order to support `read2`, whereas the `sys::anonymous_pipe` version does not do so. Thus the naming is extremely confusing.
To fix this, this PR renames the `sys::anonymous_pipe` version of `AnonPipe` to simply `Pipe`, whereas the `sys::pal::pipe` version is now called `ChildPipe`. Additionally,
* `sys::anonymous_pipe` is now also just called `sys::pipe`.
* On Windows, `sys::pal::pipe` has been moved to `sys::process` and is now called `sys::process::child_pipe`.
* On non-Windows platforms, pipe creation is now exclusively handled by `sys::pipe` and `ChildPipe` is defined as a type alias to `Pipe` within `sys::process`.
And lastly, the `read2` function (originally in `sys::pal::pipe`) is now called `read_output` and defined by `sys::process`, as (at least on Windows) it is only usable with `ChildPipe`.
Includes rust-lang/rust#146639 for convenience.
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