We rely on the GC to clear this when the GC is run on another EC than
the cache.

This fixes a bug where the gen_fields_cache could become invalid when
the last ivar was removed. Also adds more assertions.

`vm_setinstancevariable` had a codepath to try to match the inline
cache for types other than T_OBJECT, but the cache population path
in `vm_setivar_slowpath` was exclusive to T_OBJECT, so `vm_setivar_default`
would never match anything.

This commit improves `vm_setivar_slowpath` so that it is capable of
filling the cache for all types, and adds a `vm_setivar_class` codepath
for `T_CLASS` and `T_MODULE`.

`vm_setivar`, `vm_setivar_default` and `vm_setivar_class` could be unified,
but based on the very explicit `NOINLINE` I assume they were split to minimize
codesize.

```
compare-ruby: ruby 3.5.0dev (2025-08-27T14:58:58Z merge-vm-setivar-d.. 5b749d8e53) +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-08-27T16:30:31Z setivar-cache-gene.. 4fe78ff296) +PRISM [arm64-darwin24]

|                         |compare-ruby|built-ruby|
|:------------------------|-----------:|---------:|
|vm_ivar_set_on_instance  |     161.809|   164.688|
|                         |           -|     1.02x|
|vm_ivar_set_on_generic   |      58.769|   115.638|
|                         |           -|     1.97x|
|vm_ivar_set_on_class     |      70.034|   141.042|
|                         |           -|     2.01x|
```

[Bug #21547]

Followup: https://github.com/ruby/ruby/pull/14201

When adding an instance variable and the IMEMO/fields need to be
larger, we allocate a new one and clear the old one.

Since the old one may still be in other ec's cache, on a hit we must
check the IMEMO/fields isn't a stale one.

[Bug #21438]

Previously GC could trigger a table rebuild of the generic fields
st_table in the middle of calling the st_update callback. This could
cause entries to be reallocated or rearranged and the update to be for
the wrong entry.

This commit adds an assertion to make that case easier to detect, and
replaces the st_update with a separate st_lookup and st_insert.

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

Co-authored-by: John Hawthorn <john@hawthorn.email>