[Universal parser] Decouple IMEMO from rb_ast_t

This patch removes the `VALUE flags` member from the `rb_ast_t` structure making `rb_ast_t` no longer an IMEMO object.

## Background

We are trying to make the Ruby parser generated from parse.y a universal parser that can be used by other implementations such as mruby.
To achieve this, it is necessary to exclude VALUE and IMEMO from parse.y, AST, and NODE.

## Summary (file by file)

- `rubyparser.h`
  - Remove the `VALUE flags` member from `rb_ast_t`
- `ruby_parser.c` and `internal/ruby_parser.h`
  - Use TypedData_Make_Struct VALUE which wraps `rb_ast_t` `in ast_alloc()` so that GC can manage it
    - You can retrieve `rb_ast_t` from the VALUE by `rb_ruby_ast_data_get()`
  - Change the return type of `rb_parser_compile_XXXX()` functions from `rb_ast_t *` to `VALUE`
  - rb_ruby_ast_new() which internally `calls ast_alloc()` is to create VALUE vast outside ruby_parser.c
- `iseq.c` and `vm_core.h`
  - Amend the first parameter of `rb_iseq_new_XXXX()` functions from `rb_ast_body_t *` to `VALUE`
  - This keeps the VALUE of AST on the machine stack to prevent being removed by GC
- `ast.c`
  - Almost all change is replacement `rb_ast_t *ast` with `VALUE vast` (sorry for the big diff)
  - Fix `node_memsize()`
    - Now it includes `rb_ast_local_table_link`, `tokens` and script_lines
- `compile.c`, `load.c`, `node.c`, `parse.y`, `proc.c`, `ruby.c`, `template/prelude.c.tmpl`, `vm.c` and `vm_eval.c`
  - Follow-up due to the above changes
- `imemo.{c|h}`
  - If an object with `imemo_ast` appears, considers it a bug

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

node.c

@@ -304,14 +304,16 @@ rb_ast_t *
 rb_ast_new(const rb_parser_config_t *config)
 {
     node_buffer_t *nb = rb_node_buffer_new(config);
-    return config->ast_new((VALUE)nb);
+    return config->ast_new(nb);
 }
 #else
 rb_ast_t *
 rb_ast_new(void)
 {
     node_buffer_t *nb = rb_node_buffer_new();
-    return IMEMO_NEW(rb_ast_t, imemo_ast, (VALUE)nb);
+    rb_ast_t *ast = ruby_xcalloc(1, sizeof(rb_ast_t));
+    ast->node_buffer = nb;
+    return ast;
 }
 #endif
 
@@ -348,14 +350,26 @@ script_lines_free(rb_ast_t *ast, rb_parser_ary_t *script_lines)
 void
 rb_ast_free(rb_ast_t *ast)
 {
-    if (ast->node_buffer) {
+    /* TODO
+     * The current impl. of rb_ast_free() and rb_ast_dispose() is complicated.
+     * Upcoming task of changing `ast->node_buffer->config` to `ast->config`,
+     * that is based on "deIMEMO" of `rb_ast_t *`, will let us simplify the code.
+     */
+#ifdef UNIVERSAL_PARSER
+    if (ast && ast->node_buffer) {
+        void (*free_func)(void *) = xfree;
         if (ast->body.script_lines && !FIXNUM_P((VALUE)ast->body.script_lines)) {
             script_lines_free(ast, ast->body.script_lines);
             ast->body.script_lines = NULL;
         }
         rb_node_buffer_free(ast, ast->node_buffer);
         ast->node_buffer = 0;
+        free_func(ast);
     }
+#else
+    rb_ast_dispose(ast);
+    xfree(ast);
+#endif
 }
 
 static size_t
@@ -373,20 +387,63 @@ buffer_list_size(node_buffer_list_t *nb)
 size_t
 rb_ast_memsize(const rb_ast_t *ast)
 {
-    size_t size = 0;
+    size_t size = sizeof(rb_ast_t);
     node_buffer_t *nb = ast->node_buffer;
+    rb_parser_ary_t *tokens = NULL;
+    struct rb_ast_local_table_link *link = NULL;
+    rb_parser_ary_t *script_lines = ast->body.script_lines;
+
+    long i;
 
     if (nb) {
         size += sizeof(node_buffer_t);
         size += buffer_list_size(&nb->buffer_list);
+        link = nb->local_tables;
+        tokens = nb->tokens;
     }
+
+    while (link) {
+        size += sizeof(struct rb_ast_local_table_link);
+        size += link->size * sizeof(ID);
+        link = link->next;
+    }
+
+    if (tokens) {
+        size += sizeof(rb_parser_ary_t);
+        for (i = 0; i < tokens->len; i++) {
+            size += sizeof(rb_parser_ast_token_t);
+            rb_parser_ast_token_t *token = tokens->data[i];
+            size += sizeof(rb_parser_string_t);
+            size += token->str->len + 1;
+        }
+    }
+
+    if (script_lines && !FIXNUM_P((VALUE)script_lines)) {
+        size += sizeof(rb_parser_ary_t);
+        for (i = 0; i < script_lines->len; i++) {
+            size += sizeof(rb_parser_string_t);
+            size += ((rb_parser_string_t *)script_lines->data[i])->len + 1;
+        }
+    }
+
     return size;
 }
 
 void
 rb_ast_dispose(rb_ast_t *ast)
 {
-    rb_ast_free(ast);
+#ifdef UNIVERSAL_PARSER
+    // noop. See the comment in rb_ast_free().
+#else
+    if (ast && ast->node_buffer) {
+        if (ast->body.script_lines && !FIXNUM_P((VALUE)ast->body.script_lines)) {
+            script_lines_free(ast, ast->body.script_lines);
+            ast->body.script_lines = NULL;
+        }
+        rb_node_buffer_free(ast, ast->node_buffer);
+        ast->node_buffer = 0;
+    }
+#endif
 }
 
 VALUE