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)
HASUMI Hitoshi
55a402bb75