All accesses to `generic_fields_tbl_` are now encapsulated inside:

  - `rb_obj_fields`
  - `rb_obj_set_fields`
  - `rb_obj_replace_fields`

Followup: https://github.com/ruby/ruby/pull/13589

This simplify a lot of things, as we no longer need to manually
manage the memory, we can use the Read-Copy-Update pattern and
avoid numerous race conditions.

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>

This data is redundant because the shape already contains both the
length and capacity of the object's fields.

So it both waste space and create the possibility of a desync between
the two.

We also do not need to initialize everything to Qundef, this seem
to be a left-over from pre-shape instance variables.

This makes `RBobject` `4B` larger on 32 bit systems
but simplifies the implementation a lot.

[Feature #21353]

Co-authored-by: Jean Boussier <byroot@ruby-lang.org>

Ivars will longer be the only thing stored inline
via shapes, so keeping the `iv_index` and `ivptr` names
would be confusing.

Instance variables won't be the only thing stored inline
via shapes, so keeping the `ivptr` name would be confusing.

`field` encompass anything that can be stored in a VALUE array.

Similarly, `gen_ivtbl` becomes `gen_fields_tbl`.

when the RUBY_FREE_ON_SHUTDOWN environment variable is set, manually free memory at shutdown.

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>
Co-authored-by: Peter Zhu <peter@peterzhu.ca>

On 32-bit systems, we must store the shape ID in the gen_ivtbl to not
lose the shape. If we directly store the ST table into the generic
ivar table, then we lose the shape. This makes it impossible to
determine the shape of the object and whether it is too complex or not.

so that now shape can happily include gc.h

This reverts commit 9a6803c90b817f70389cae10d60b50ad752da48f.