Fix `hex_color!` macro by re-exporting `color_hex` crate from `ecolor` (#4372)

The `hex_color!` macro currently makes an unqualified reference to the
`color_hex` crate, which users may not have in their `Cargo.toml`. This
PR re-exports `color_hex` from the root of `ecolor` and changes the
macro to use the re-exported path.

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* Closes https://github.com/emilk/egui/issues/2644

crates/ecolor/src/hex_color_macro.rs

@@ -13,7 +13,7 @@
 #[macro_export]
 macro_rules! hex_color {
     ($s:literal) => {{
-        let array = color_hex::color_from_hex!($s);
+        let array = $crate::color_hex::color_from_hex!($s);
         if array.len() == 3 {
             $crate::Color32::from_rgb(array[0], array[1], array[2])
         } else {

crates/ecolor/src/lib.rs

@@ -24,6 +24,9 @@ pub use hsva::*;
 
 #[cfg(feature = "color-hex")]
 mod hex_color_macro;
+#[cfg(feature = "color-hex")]
+#[doc(hidden)]
+pub use color_hex;
 
 mod rgba;
 pub use rgba::*;