Add custom state.

src/eval/eval_context.rs

@@ -60,6 +60,18 @@ impl<'s, 'ps, 'm, 'pm, 'pt> EvalContext<'_, 's, 'ps, 'm, 'pm, '_, '_, 'pt> {
     pub fn iter_imports(&self) -> impl Iterator<Item = (&str, &Module)> {
         self.global.iter_imports()
     }
+    /// Custom state kept in a [`Dynamic`].
+    #[inline(always)]
+    #[must_use]
+    pub const fn tag(&self) -> &Dynamic {
+        &self.global.tag
+    }
+    /// Mutable reference to the custom state kept in a [`Dynamic`].
+    #[inline(always)]
+    #[must_use]
+    pub fn tag_mut(&mut self) -> &mut Dynamic {
+        &mut self.global.tag
+    }
     /// _(internals)_ The current [`GlobalRuntimeState`].
     /// Exported under the `internals` feature only.
     #[cfg(feature = "internals")]