Relax tests for Windows dos device names
Windows 11 no longer turn paths ending with dos device names into device paths.
E.g. `C:\path\to\COM1.txt` used to get turned into `\\.\COM1`. Whereas now this path is left as is.
Note though that if the given path is an exact (case-insensitive) match for the string `COM1` then it'll still be converted to `\\.\COM1`.
This updates the standard library's documentation to use the new syntax. The
documentation is worthwhile to update as it should be more idiomatic
(particularly for features like this, which are nice for users to get acquainted
with). The general codebase is likely more hassle than benefit to update: it'll
hurt git blame, and generally updates can be done by folks updating the code if
(and when) that makes things more readable with the new format.
A few places in the compiler and library code are updated (mostly just due to
already having been done when this commit was first authored).
Windows 11 no longer turn paths ending with dos device names into device paths.
E.g. `C:\path\to\COM1.txt` used to get turned into `\\.\COM1`. Whereas now the path is left as is.
* the logic only checked for / but not for \
* verbatim paths shouldn't skip items at all since they don't get normalized
* the extra branches get optimized out on unix since is_sep_byte is a trivial comparison and is_verbatim is always-false
* tests lacked windows coverage for these cases
That lead to equal paths not having equal hashes and to unnecessary collisions.
add file_prefix method to std::path
This is an initial implementation of `std::path::Path::file_prefix`. It is effectively a "left" variant of the existing [`file_stem`](https://doc.rust-lang.org/std/path/struct.Path.html#method.file_stem) method. An illustration of the difference is
```rust
use std::path::Path;
let path = Path::new("foo.tar.gz");
assert_eq!(path.file_stem(), Some("foo.tar"));
assert_eq!(path.file_prefix(), Some("foo"));
```
In my own development, I generally find I almost always want the prefix, rather than the stem, so I thought it might be best to suggest it's addition to libstd.
Of course, as this is my first contribution, I expect there is probably more work that needs to be done. Additionally, if the libstd team feel this isn't appropriate then so be it.
There has been some [discussion about this on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/file_lstem/near/238076313) and a user there suggested I open a PR to see whether someone in the libstd team thinks it is worth pursuing.
Refactor `get_first_two_components` to `get_next_component`.
Fixes the following behaviour of `parse_prefix`:
- series of separator bytes in a prefix are correctly parsed as a single separator
- device namespace prefixes correctly recognize both `\\` and `/` as separators
