Deprecate ahash re-exports (#4979)

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* Closes <https://github.com/emilk/egui/issues/3482>
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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/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;