Rollup merge of #23010 - alexcrichton:deprecate-some-old-io, r=aturon

 The new `io` module has had some time to bake and this commit stabilizes some of
the utilities associated with it. This commit also deprecates a number of
`std::old_io::util` functions and structures.

These items are now `#[stable]`

* `Cursor`
* `Cursor::{new, into_inner, get_ref, get_mut, position, set_position}`
* Implementations of I/O traits for `Cursor<T>`
* Delegating implementations of I/O traits for references and `Box` pointers
* Implementations of I/O traits for primitives like slices and `Vec<T>`
* `ReadExt::bytes`
* `Bytes` (and impls)
* `ReadExt::chain`
* `Chain` (and impls)
* `ReadExt::take` (and impls)
* `BufReadExt::lines`
* `Lines` (and impls)
* `io::copy`
* `io::{empty, Empty}` (and impls)
* `io::{sink, Sink}` (and impls)
* `io::{repeat, Repeat}` (and impls)

These items remain `#[unstable]`

* Core I/O traits. These may want a little bit more time to bake along with the
  commonly used methods like `read_to_end`.
* `BufReadExt::split` - this function may be renamed to not conflict with
  `SliceExt::split`.
* `Error` - there are a number of questions about its representation,
  `ErrorKind`, and usability.

These items are now `#[deprecated]` in `old_io`

* `LimitReader` - use `take` instead
* `NullWriter` - use `io::sink` instead
* `ZeroReader` - use `io::repeat` instead
* `NullReader` - use `io::empty` instead
* `MultiWriter` - use `broadcast` instead
* `ChainedReader` - use `chain` instead
* `TeeReader` - use `tee` instead
* `copy` - use `io::copy` instead

[breaking-change]

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