Reproduction script:

```
require "ripper"

10.times do
  20_000.times do
    Ripper.parse("")
  end

  puts `ps -o rss= -p #{$$}`
end
```

Before:

```
28032
34432
40704
47232
53632
60032
66432
72832
79232
85632
```

After:

```
21760
21760
21760
21760
21760
21760
21760
21760
21760
21760
```

All kind of AST nodes use same struct RNode, which has u1, u2, u3 union members
for holding different kind of data.
This has two problems.

1. Low flexibility of data structure

Some nodes, for example NODE_TRUE, don’t use u1, u2, u3. On the other hand,
NODE_OP_ASGN2 needs more than three union members. However they use same
structure definition, need to allocate three union members for NODE_TRUE and
need to separate NODE_OP_ASGN2 into another node.
This change removes the restriction so make it possible to
change data structure by each node type.

2. No compile time check for union member access

It’s developer’s responsibility for using correct member for each node type when it’s union.
This change clarifies which node has which type of fields and enables compile time check.

This commit also changes node_buffer_elem_struct buf management to handle
different size data with alignment.

NODE_ARGS, NODE_ARYPTN, NODE_FNDPTN manage memory of their
structure by imemo tmpbuf Object.
However rb_ast_struct has reference to NODE. Then these
memory can be freed directly when rb_ast_struct is freed.

This commit reduces parser's dependency on CRuby functions.

This commit reduces dependency to CRuby object.

Introduce Universal Parser mode for the parser.
This commit includes these changes:

* Introduce `UNIVERSAL_PARSER` macro. All of CRuby related functions
  are passed via `struct rb_parser_config_struct` when this macro is enabled.
* Add CI task with 'cppflags=-DUNIVERSAL_PARSER' for ubuntu.

98637d421dbe8bcf86cc2effae5e26bb96a6a4da changes the name of
the function. However this function is exported as global,
then change the name to origin one for keeping compatibility.