Rework tree generation mode and API

Store tree nodes in congruent, compact arena array.
Define handle types to refer to tree nodes rather than pointers to
structure instances.
Free tree with context.

CHANGELOG.md

@@ -1,3 +1,30 @@
+## v5.0.0
+
+### API Changes
+
+- Tree generation mode now stores all tree nodes in a compact arena owned by
+  the parser context (a flat node array plus a shared child-link array).
+  This replaces the previous design of one heap allocation per node with
+  layout-punned typed structs.
+- Tree nodes are now referenced by lightweight handles rather than pointers.
+  `p_result()` and the field accessors return handle values.
+- The whole tree is freed together with the context by `p_context_delete()`.
+  The `p_tree_delete()` / `p_tree_delete_XXX()` functions have been removed;
+  tree node handles are only valid while the context is alive.
+- Tree node field access changed per target language:
+  - C: per-field accessor functions (e.g. `p_Start_pItems(node)`) plus tree
+    walk macros (e.g. `p_tree_walk_Start(node, pItems, pItem, pToken1, token)`),
+    and generic accessors `p_node_valid()`, `p_node_position()`,
+    `p_node_end_position()`, `p_node_n_fields()`, `p_node_data()`, `p_node_id()`.
+  - C++: handle methods called with `()` (e.g. `node.pItems().pToken1().token()`),
+    plus the same C-style functions/macros for convenience.
+  - D: `@property` accessors preserving the previous field-access syntax
+    (e.g. `node.pItems.pToken1.token`); null checks use `.valid` instead of
+    `is null`.
+- Tree-mode parser rule user code: `$$` and `$1` etc. now yield node handles.
+  Reference child fields through the target-language accessors described above
+  rather than through struct pointer members.
+
 ## v4.8.1
 
 ### Fixes