Implement gen_fields_tbl cache

When accessing an object ivars, it's not rare to access the same
object's ivar quickly after.

For instance from enumerator.c:

```c
    enumerator = rb_obj_alloc(rb_cEnumerator);
    rb_ivar_set(enumerator, id_sliceafter_enum, enumerable);
    rb_ivar_set(enumerator, id_sliceafter_pat, pat);
    rb_ivar_set(enumerator, id_sliceafter_pred, pred);
```

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.

Author: byroot

variable.c

@@ -1238,9 +1238,19 @@ rb_obj_fields(VALUE obj, ID field_name)
             }
             // fall through
           default:
-            RB_VM_LOCKING() {
-                if (!st_lookup(generic_fields_tbl_, (st_data_t)obj, (st_data_t *)&fields_obj)) {
-                    rb_bug("Object is missing entry in generic_fields_tbl");
+            {
+                rb_execution_context_t *ec = GET_EC();
+                if (ec->gen_fields_cache.obj == obj) {
+                    fields_obj = ec->gen_fields_cache.fields_obj;
+                }
+                else {
+                    RB_VM_LOCKING() {
+                        if (!st_lookup(generic_fields_tbl_, (st_data_t)obj, (st_data_t *)&fields_obj)) {
+                            rb_bug("Object is missing entry in generic_fields_tbl");
+                        }
+                    }
+                    ec->gen_fields_cache.fields_obj = fields_obj;
+                    ec->gen_fields_cache.obj = obj;
                 }
             }
         }
@@ -1261,8 +1271,15 @@ rb_free_generic_ivar(VALUE obj)
             }
             // fall through
           default:
-            RB_VM_LOCKING() {
-                st_delete(generic_fields_tbl_no_ractor_check(), &key, &value);
+            {
+                rb_execution_context_t *ec = GET_EC();
+                if (ec->gen_fields_cache.obj == obj) {
+                    ec->gen_fields_cache.obj = Qundef;
+                    ec->gen_fields_cache.fields_obj = Qundef;
+                }
+                RB_VM_LOCKING() {
+                    st_delete(generic_fields_tbl_no_ractor_check(), &key, &value);
+                }
             }
         }
         RBASIC_SET_SHAPE_ID(obj, ROOT_SHAPE_ID);
@@ -1286,10 +1303,18 @@ rb_obj_set_fields(VALUE obj, VALUE fields_obj, ID field_name, VALUE original_fie
             }
             // fall through
           default:
-            RB_VM_LOCKING() {
-                st_insert(generic_fields_tbl_, (st_data_t)obj, (st_data_t)fields_obj);
+            {
+                RB_VM_LOCKING() {
+                    st_insert(generic_fields_tbl_, (st_data_t)obj, (st_data_t)fields_obj);
+                }
+                RB_OBJ_WRITTEN(obj, original_fields_obj, fields_obj);
+
+                rb_execution_context_t *ec = GET_EC();
+                if (ec->gen_fields_cache.fields_obj != fields_obj) {
+                    ec->gen_fields_cache.obj = obj;
+                    ec->gen_fields_cache.fields_obj = fields_obj;
+                }
             }
-            RB_OBJ_WRITTEN(obj, original_fields_obj, fields_obj);
         }
 
         if (original_fields_obj) {

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);

vm_core.h

@@ -1070,6 +1070,11 @@ struct rb_execution_context_struct {
 
     VALUE private_const_reference;
 
+    struct {
+        VALUE obj;
+        VALUE fields_obj;
+    } gen_fields_cache;
+
     /* for GC */
     struct {
         VALUE *stack_start;