Add GDB pretty-printer for zend_ast

[arnaud-lb]

This builds on top of #13463 to add GDB pretty-printers for zend_ast. Since ASTs contain zval, I've also included a minimal zval pretty printer.

Similarly to #13463, with these printers I'm trying to not hide the internals of the pretty printed types as this would be inconvenient when debugging. I just use to_string() to print a small string representation of the type when possible, and children() to help GDB walk the struct.

As zend_ast are deep structures, these settings are useful:

(gdb) set print pretty on
(gdb) set print max-depth 2

The first one enables indentation.

(gdb) p ast
$1 = (zend_ast *) 0x7ffff7a87180

(gdb) p /r *ast
$2 = {
  kind = 67,
  attr = 0,
  lineno = 1,
  child = {0x1}
}

(gdb) p *ast
$3 = ((zend_ast_decl*)0x7ffff7a87180) = {
  kind = ZEND_AST_FUNC_DECL,
  attr = 0,
  start_lineno = 1,
  end_lineno = 1,
  flags = 0,
  doc_comment = 0x0,
  name = "some_function_name",
  child[0] = ((zend_ast_list*)0x7ffff7a870c0) = {
    kind = ZEND_AST_PARAM_LIST,
    attr = 0,
    lineno = 1,
    children = 2,
    child[0] = ((zend_ast*)0x7ffff7a87090) = {...},
    child[1] = ((zend_ast*)0x7ffff7a87120) = {...}
  },
  child[1] = 0x0,
  child[2] = ((zend_ast_list*)0x7ffff7a87150) = {
    kind = ZEND_AST_STMT_LIST,
    attr = 0,
    lineno = 1,
    children = 0
  },
  child[3] = 0x0,
  child[4] = 0x0
}

[arnaud-lb]

I'm going to merge this in a few days unless there are objections