Add line_count field to rb_ast_body_t

This patch adds `int line_count` field to `rb_ast_body_t` structure.
Instead, we no longer cast `script_lines` to Fixnum.

## Background

Ref https://github.com/ruby/ruby/pull/10618

In the PR above, we have decoupled IMEMO from `rb_ast_t`.
This means we could lift the five-words-restriction of the structure
that forced us to unionize `rb_ast_t *` and `FIXNUM` in one field.

## Relating refactor

- Remove the second parameter of `rb_ruby_ast_new()` function

## Attention

I will remove a code that assigns -1 to line_count, in `rb_binding_add_dynavars()`
of vm.c, because I don't think it is necessary.
But I will make another PR for this so that we can atomically revert
in case I was wrong (See the comment on the code)

ast.c

@@ -802,7 +802,6 @@ ast_node_script_lines(rb_execution_context_t *ec, VALUE self)
     TypedData_Get_Struct(self, struct ASTNodeData, &rb_node_type, data);
     ast = rb_ruby_ast_data_get(data->vast);
     rb_parser_ary_t *ret = ast->body.script_lines;
-    if (!ret || FIXNUM_P((VALUE)ret)) return Qnil;
     return rb_parser_build_script_lines_from(ret);
 }