mruby-compiler: add infrastructure for variable-sized ast nodes

This commit introduces the core infrastructure for variable-sized AST
nodes, designed to improve memory efficiency. The previous fixed-size
nodes are replaced by nodes that can store data inline, such as
strings and integers, reducing pointer indirection and memory overhead.

Key changes include:
- A generic variable-sized node header (`mrb_ast_var_header`).
- A size-class-based memory allocation system for these nodes.
- Implementation of variable-sized nodes for core types: symbols,
  strings, integers, and variables (lvar, gvar, ivar, cvar).
- Integration into the parser and code generator, controlled by a
  feature flag.
- Centralized and improved type-casting macros for AST nodes.

Co-authored-by: Claude <noreply@anthropic.com>

include/mruby/compile.h

@@ -33,6 +33,7 @@ typedef struct mrb_ccontext {
   mrb_bool keep_lv:1;
   mrb_bool no_optimize:1;
   mrb_bool no_ext_ops:1;
+  mrb_bool use_variable_nodes:1;  /* Enable variable-sized AST nodes */
   const struct RProc *upper;
 
   size_t parser_nerr;
@@ -133,6 +134,11 @@ struct mrb_parser_state {
   uint16_t filename_table_length;
   uint16_t current_filename_index;
 
+  /* Variable-sized node management */
+  struct mrb_ast_node *var_free_lists[5];  /* Free lists by size class (SIZE_CLASS_COUNT) */
+  size_t var_alloc_counts[5];              /* Allocation tracking by size class */
+  size_t var_total_allocated;              /* Total variable node memory */
+  mrb_bool var_nodes_enabled:1;            /* Feature flag for gradual rollout */
   mrb_ast_node *nvars;
 };