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>

This follows the same approach used for attr_reader/attr_writer in
2d98593bf54a37397c6e4886ccc7e3654c2eaf85, skipping the checking for
tracing after the first call using the call cache, and clearing the
call cache when tracing is turned on/off.

Fixes [Bug #18886]