Speed up Ractors for Variable Width Allocation

This commit adds a Ractor cache for every size pool. Previously, all VWA
allocated objects used the slowpath and locked the VM.

On a micro-benchmark that benchmarks String allocation:

VWA turned off:
  29.196591   0.889709  30.086300 (  9.434059)

VWA before this commit:
  29.279486  41.477869  70.757355 ( 12.527379)

VWA after this commit:
  16.782903   0.557117  17.340020 (  4.255603)

1 files changed, 3 insertions, 1 deletions

diff --git a/class.c b/class.c
index 6ad64a6efe..f1e8953f81 100644
--- a/class.c
+++ b/class.c
@@ -203,7 +203,9 @@ class_alloc(VALUE flags, VALUE klass)
 
     RVARGC_NEWOBJ_OF(obj, struct RClass, klass, (flags & T_MASK) | FL_PROMOTED1 /* start from age == 2 */ | (RGENGC_WB_PROTECTED_CLASS ? FL_WB_PROTECTED : 0), alloc_size);
 
-#if !USE_RVARGC
+#if USE_RVARGC
+    memset(RCLASS_EXT(obj), 0, sizeof(rb_classext_t));
+#else
     obj->ptr = ZALLOC(rb_classext_t);
 #endif