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
2d98593bf5, 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]
