Deprecate `ahash` re-exports (#4979)

<!--
Please read the "Making a PR" section of
[`CONTRIBUTING.md`](https://github.com/emilk/egui/blob/master/CONTRIBUTING.md)
before opening a Pull Request!

* Keep your PR:s small and focused.
* The PR title is what ends up in the changelog, so make it descriptive!
* If applicable, add a screenshot or gif.
* If it is a non-trivial addition, consider adding a demo for it to
`egui_demo_lib`, or a new example.
* Do NOT open PR:s from your `master` branch, as that makes it hard for
maintainers to test and add commits to your PR.
* Remember to run `cargo fmt` and `cargo clippy`.
* Open the PR as a draft until you have self-reviewed it and run
`./scripts/check.sh`.
* When you have addressed a PR comment, mark it as resolved.

Please be patient! I will review your PR, but my time is limited!
-->
* Closes <https://github.com/emilk/egui/issues/3482>
* [x] I have followed the instructions in the PR template

Regarding the statement in #3482,
https://docs.rs/egui/latest/egui/layers/struct.GraphicLayers.html#method.drain
accepts a HashMap, but not sure if that is enough to keep the re-export.

crates/egui/src/lib.rs

@@ -415,6 +415,7 @@ mod callstack;
 #[cfg(feature = "accesskit")]
 pub use accesskit;
 
+#[deprecated = "Use the ahash crate directly."]
 pub use ahash;
 
 pub use epaint;

crates/egui_extras/src/loaders/gif_loader.rs

@@ -1,5 +1,5 @@
+use ahash::HashMap;
 use egui::{
-    ahash::HashMap,
     decode_gif_uri, has_gif_magic_header,
     load::{BytesPoll, ImageLoadResult, ImageLoader, ImagePoll, LoadError, SizeHint},
     mutex::Mutex,

crates/epaint/src/lib.rs

@@ -67,7 +67,9 @@ pub use tessellator::tessellate_shapes;
 pub use ecolor::{Color32, Hsva, HsvaGamma, Rgba};
 pub use emath::{pos2, vec2, Pos2, Rect, Vec2};
 
+#[deprecated = "Use the ahash crate directly."]
 pub use ahash;
+
 pub use ecolor;
 pub use emath;