It's not rare for structs to have additional ivars, hence are one
of the most common, if not the most common type in the `gen_fields_tbl`.

This can cause Ractor contention, but even in single ractor mode
means having to do a hash lookup to access the ivars, and increase
GC work.

Instead, unless the struct is perfectly right sized, we can store
a reference to the associated IMEMO/fields object right after the
last struct member.

```
compare-ruby: ruby 3.5.0dev (2025-08-06T12:50:36Z struct-ivar-fields-2 9a30d141a1) +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-08-06T12:57:59Z struct-ivar-fields-2 2ff3ec237f) +PRISM [arm64-darwin24]
warming up.....

|                      |compare-ruby|built-ruby|
|:---------------------|-----------:|---------:|
|member_reader         |    590.317k|  579.246k|
|                      |       1.02x|         -|
|member_writer         |    543.963k|  527.104k|
|                      |       1.03x|         -|
|member_reader_method  |    213.540k|  213.004k|
|                      |       1.00x|         -|
|member_writer_method  |    192.657k|  191.491k|
|                      |       1.01x|         -|
|ivar_reader           |    403.993k|  569.915k|
|                      |           -|     1.41x|
```

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

The benchmark results show that this feature has either a positive or
no impact on performance. The memory usage is also mostly unchanged,
except in hexapdf, where there is a decrease in RSS.

--------------  -----------  ----------  ---------  -----------  ----------  ---------  --------------  -------------
bench           master (ms)  stddev (%)  RSS (MiB)  branch (ms)  stddev (%)  RSS (MiB)  branch 1st itr  master/branch
activerecord    70.8         2.2         56.0       71.7         2.2         56.0       0.99            0.99
erubi_rails     20.5         13.6        94.7       20.5         14.3        94.2       0.93            1.00
hexapdf         2541.0       0.7         212.8      2544.4       0.7         203.4      1.00            1.00
liquid-c        65.6         0.3         38.9       65.3         0.3         38.9       1.01            1.01
liquid-compile  63.7         0.3         34.6       61.1         0.2         34.6       1.04            1.04
liquid-render   163.1        0.1         37.1       163.3        0.1         37.1       1.00            1.00
mail            139.3        0.1         50.5       137.0        0.1         50.1       0.99            1.02
psych-load      2065.7       0.1         36.9       2068.2       0.1         37.3       1.00            1.00
railsbench      2034.6       0.5         103.9      2031.9       0.5         103.8      1.02            1.00
ruby-lsp        65.3         3.1         89.8       66.2         3.0         89.7       1.01            0.99
sequel          73.2         1.0         40.3       73.4         1.0         40.3       1.00            1.00
--------------  -----------  ----------  ---------  -----------  ----------  ---------  --------------  -------------

RB_OBJ_WRITE already exists in rgengc.h, so we shouldn't redefine it in
gc.h.

These contents are purely implementation details, not worth appearing in
CAPI documents. [ci skip]

Remove all usages of RCAST() so that the header file can be excluded
from ruby/ruby.h's dependency.

To fix build failures.

This shall fix compile errors.