doc: Update output of `rustc --version`
Remove core::any::AnyPrivate

[Previously](https://github.com/rust-lang/rust/commit/e5da6a71a6a0b46dd3630fc8326e6d5906a1fde6),
the `Any` trait was split into a private portion and an (empty) public
portion, in order to hide the implementation strategy used for
downcasting. However, the [new
rules](https://github.com/rust-lang/rust/commit/e9ad12c0cae5c43ada6641c7dc840a0fbe5010a2)
for privacy forbid `AnyPrivate` from actually being private.

This patch thus reverts the introduction of `AnyPrivate`.

Although this is unlikely to break any real code, it removes a public
trait and is therefore a:

[breaking-change]
Revise DST test to not require duplicate errors.
Use slice syntax instead of slice_to, etc.
Put slicing syntax behind a feature gate.

[breaking-change]

If you are using slicing syntax you will need to add #![feature(slicing_syntax)] to your crate.
Rename slice::Slice
Rename slicing methods
Reinstate AsSlice impls for Option and Result
library-level docs for collections
add missing btree re-exports
auto merge of #17745 : aturon/rust/revert-any-private, r=alexcrichton

[Previously](https://github.com/rust-lang/rust/commit/e5da6a71a6a0b46dd3630fc8326e6d5906a1fde6), the `Any` trait was split into a private portion and an (empty) public portion, in order to hide the implementation strategy used for downcasting. However, the [new rules](https://github.com/rust-lang/rust/commit/e9ad12c0cae5c43ada6641c7dc840a0fbe5010a2) for privacy forbid `AnyPrivate` from actually being private.

This patch thus reverts the introduction of `AnyPrivate`.

Although this is unlikely to break any real code, it removes a public trait and is therefore a:

[breaking-change]
auto merge of #17807 : nick29581/rust/slice6, r=aturon

r? @aturon
auto merge of #17802 : Gankro/rust/collection-docs-redux, r=aturon

Adds a high-level discussion of "what collection should you use for what", as well as some general discussion of correct/efficient usage of the capacity, iterator, and entry APIs.

Still building docs to confirm this renders right and the examples are good, but the content can be reviewed now.
auto merge of #17832 : brson/rust/updateversion, r=alexcrichton