Implement gen_fields_tbl cache

There is a high likelyhood that `rb_obj_fields` is called
consecutively for the same object.

If we keep a cache of the last IMEMO/fields we interacted with,
we can save having to lookup the `gen_fields_tbl`, synchronize
the VM lock, etc.

On yjit-bench's, I instrumented the hit rate of this cache at:

  - `shipit`: 38%, with 111k hits.
  - `lobsters`: 59%, with 367k hits.
  - `rubocop`: 100% with only 300 hits.

I also ran a micro-benchmark which shows that ivar access is:

  - 1.25x faster when the cache is hit in single ractor mode.
  - 2x faster when the cache is hit in multi ractor mode.
  - 1.06x slower when the cache miss in single ractor mode.
  - 1.01x slower when the cache miss in multi ractor mode.

```yml
prelude: |
  class GenIvar < Array
    def initialize(...)
      super
      @iv = 1
    end

    attr_reader :iv
  end

  a = GenIvar.new
  b = GenIvar.new
benchmark:
  hit: a.iv; a.iv; a.iv; a.iv; a.iv; a.iv; a.iv; a.iv; a.iv; a.iv; a.iv; a.iv; a.iv; a.iv; a.iv; a.iv; a.iv; a.iv; a.iv; a.iv;
  miss: a.iv; b.iv; a.iv; b.iv; a.iv; b.iv; a.iv; b.iv; a.iv; b.iv; a.iv; b.iv; a.iv; b.iv; a.iv; b.iv; a.iv; b.iv; a.iv; b.iv;
```

Single ractor:
```
compare-ruby: ruby 3.5.0dev (2025-08-12T02:14:57Z master 428937a536) +YJIT +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-08-12T09:25:35Z gen-fields-cache 9456c35893) +YJIT +PRISM [arm64-darwin24]
warming up..

|      |compare-ruby|built-ruby|
|:-----|-----------:|---------:|
|hit   |      4.090M|    5.121M|
|      |           -|     1.25x|
|miss  |      3.756M|    3.534M|
|      |       1.06x|         -|
```

Multi-ractor:
```
compare-ruby: ruby 3.5.0dev (2025-08-12T02:14:57Z master 428937a536) +YJIT +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-08-12T09:25:35Z gen-fields-cache 9456c35893) +YJIT +PRISM [arm64-darwin24]
warming up..

|      |compare-ruby|built-ruby|
|:-----|-----------:|---------:|
|hit   |      2.205M|    4.460M|
|      |           -|     2.02x|
|miss  |      2.117M|    2.094M|
|      |       1.01x|         -|
```

vm.c

@@ -3441,6 +3441,9 @@ rb_execution_context_update(rb_execution_context_t *ec)
     }
 
     ec->storage = rb_gc_location(ec->storage);
+
+    ec->gen_fields_cache.obj = rb_gc_location(ec->gen_fields_cache.obj);
+    ec->gen_fields_cache.fields_obj = rb_gc_location(ec->gen_fields_cache.fields_obj);
 }
 
 static enum rb_id_table_iterator_result
@@ -3505,6 +3508,9 @@ rb_execution_context_mark(const rb_execution_context_t *ec)
     rb_gc_mark(ec->private_const_reference);
 
     rb_gc_mark_movable(ec->storage);
+
+    rb_gc_mark_weak((VALUE *)&ec->gen_fields_cache.obj);
+    rb_gc_mark_weak((VALUE *)&ec->gen_fields_cache.fields_obj);
 }
 
 void rb_fiber_mark_self(rb_fiber_t *fib);