Enhance keep_tokens option for RubyVM::AbstractSyntaxTree parsing methods

Implementation for Language Server Protocol (LSP) sometimes needs token information.
For example both `m(1)` and `m(1, )` has same AST structure other than node locations
then it's impossible to check the existence of `,` from AST. However in later case,
it might be better to suggest variables list for the second argument.
Token information is important for such case.

This commit adds these methods.

* Add `keep_tokens` option for `RubyVM::AbstractSyntaxTree.parse`, `.parse_file` and `.of`
* Add `RubyVM::AbstractSyntaxTree::Node#tokens` which returns tokens for the node including tokens for descendants nodes.
* Add `RubyVM::AbstractSyntaxTree::Node#all_tokens` which returns all tokens for the input script regardless the receiver node.

[Feature #19070]

Impacts on memory usage and performance are below:

Memory usage:

```
$ cat test.rb
root = RubyVM::AbstractSyntaxTree.parse_file(File.expand_path('../test/ruby/test_keyword.rb', __FILE__), keep_tokens: true)

$ /usr/bin/time -f %Mkb /usr/local/bin/ruby -v
ruby 3.2.0dev (2022-11-19T09:41:54Z 19070-keep_tokens d3af1b8057) [x86_64-linux]
11408kb

# keep_tokens :false
$ /usr/bin/time -f %Mkb /usr/local/bin/ruby test.rb
17508kb

# keep_tokens :true
$ /usr/bin/time -f %Mkb /usr/local/bin/ruby test.rb
30960kb
```

Performance:

```
$ cat ../ast_keep_tokens.yml
prelude: |
  src = <<~SRC
    module M
      class C
        def m1(a, b)
          1 + a + b
        end
      end
    end
  SRC
benchmark:
  without_keep_tokens: |
    RubyVM::AbstractSyntaxTree.parse(src, keep_tokens: false)
  with_keep_tokens: |
    RubyVM::AbstractSyntaxTree.parse(src, keep_tokens: true)

$ make benchmark COMPARE_RUBY="./ruby" ARGS=../ast_keep_tokens.yml
/home/kaneko.y/.rbenv/shims/ruby --disable=gems -rrubygems -I../benchmark/lib ../benchmark/benchmark-driver/exe/benchmark-driver \
            --executables="compare-ruby::./ruby -I.ext/common --disable-gem" \
            --executables="built-ruby::./miniruby -I../lib -I. -I.ext/common  ../tool/runruby.rb --extout=.ext  -- --disable-gems --disable-gem" \
            --output=markdown --output-compare -v ../ast_keep_tokens.yml
compare-ruby: ruby 3.2.0dev (2022-11-19T09:41:54Z 19070-keep_tokens d3af1b8057) [x86_64-linux]
built-ruby: ruby 3.2.0dev (2022-11-19T09:41:54Z 19070-keep_tokens d3af1b8057) [x86_64-linux]
warming up..

|                     |compare-ruby|built-ruby|
|:--------------------|-----------:|---------:|
|without_keep_tokens  |     21.659k|   21.303k|
|                     |       1.02x|         -|
|with_keep_tokens     |      6.220k|    5.691k|
|                     |       1.09x|         -|
```

node.c

@@ -1161,6 +1161,12 @@ struct node_buffer_struct {
     node_buffer_list_t markable;
     struct rb_ast_local_table_link *local_tables;
     VALUE mark_hash;
+    // - id (sequence number)
+    // - token_type
+    // - text of token
+    // - location info
+    // Array, whose entry is array
+    VALUE tokens;
 };
 
 static void
@@ -1187,6 +1193,7 @@ rb_node_buffer_new(void)
     init_node_buffer_list(&nb->markable, (node_buffer_elem_t*)((size_t)nb->unmarkable.head + bucket_size));
     nb->local_tables = 0;
     nb->mark_hash = Qnil;
+    nb->tokens = Qnil;
     return nb;
 }
 
@@ -1418,7 +1425,10 @@ rb_ast_update_references(rb_ast_t *ast)
 void
 rb_ast_mark(rb_ast_t *ast)
 {
-    if (ast->node_buffer) rb_gc_mark(ast->node_buffer->mark_hash);
+    if (ast->node_buffer) {
+        rb_gc_mark(ast->node_buffer->mark_hash);
+        rb_gc_mark(ast->node_buffer->tokens);
+    }
     if (ast->body.compile_option) rb_gc_mark(ast->body.compile_option);
     if (ast->node_buffer) {
         node_buffer_t *nb = ast->node_buffer;
@@ -1477,3 +1487,15 @@ rb_ast_add_mark_object(rb_ast_t *ast, VALUE obj)
     }
     rb_hash_aset(ast->node_buffer->mark_hash, obj, Qtrue);
 }
+
+VALUE
+rb_ast_tokens(rb_ast_t *ast)
+{
+    return ast->node_buffer->tokens;
+}
+
+void
+rb_ast_set_tokens(rb_ast_t *ast, VALUE tokens)
+{
+    RB_OBJ_WRITE(ast, &ast->node_buffer->tokens, tokens);
+}