Mark iseq structs with rb_gc_mark_movable

Using rb_gc_mark_movable and a reference update function, we can make
instruction sequences movable in memory, and avoid pinning compiled iseqs.

```
require "objspace"
iseqs = []
GC.disable
50_000.times do
  iseqs << RubyVM::InstructionSequence.compile("")
end
GC.enable
GC.compact
p ObjectSpace.dump_all(output: :string).lines.grep(/"pinned":true/).count
```

Co-authored-by: Peter Zhu <peter@peterzhu.ca>

iseq.c

@@ -1386,18 +1386,30 @@ rb_iseq_remove_coverage_all(void)
 static void
 iseqw_mark(void *ptr)
 {
-    rb_gc_mark((VALUE)ptr);
+    rb_gc_mark_movable(*(VALUE *)ptr);
 }
 
 static size_t
 iseqw_memsize(const void *ptr)
 {
-    return rb_iseq_memsize((const rb_iseq_t *)ptr);
+    return rb_iseq_memsize(*(const rb_iseq_t **)ptr);
+}
+
+static void
+iseqw_ref_update(void *ptr)
+{
+    VALUE *vptr = ptr;
+    *vptr = rb_gc_location(*vptr);
 }
 
 static const rb_data_type_t iseqw_data_type = {
     "T_IMEMO/iseq",
-    {iseqw_mark, NULL, iseqw_memsize,},
+    {
+        iseqw_mark,
+        RUBY_TYPED_DEFAULT_FREE,
+        iseqw_memsize,
+        iseqw_ref_update,
+    },
     0, 0, RUBY_TYPED_FREE_IMMEDIATELY|RUBY_TYPED_WB_PROTECTED
 };
 
@@ -1408,11 +1420,9 @@ iseqw_new(const rb_iseq_t *iseq)
         return iseq->wrapper;
     }
     else {
-        union { const rb_iseq_t *in; void *out; } deconst;
-        VALUE obj;
-        deconst.in = iseq;
-        obj = TypedData_Wrap_Struct(rb_cISeq, &iseqw_data_type, deconst.out);
-        RB_OBJ_WRITTEN(obj, Qundef, iseq);
+        rb_iseq_t **ptr;
+        VALUE obj = TypedData_Make_Struct(rb_cISeq, rb_iseq_t *, &iseqw_data_type, ptr);
+        RB_OBJ_WRITE(obj, ptr, iseq);
 
         /* cache a wrapper object */
         RB_OBJ_WRITE((VALUE)iseq, &iseq->wrapper, obj);
@@ -1736,7 +1746,9 @@ iseqw_s_compile_option_get(VALUE self)
 static const rb_iseq_t *
 iseqw_check(VALUE iseqw)
 {
-    rb_iseq_t *iseq = DATA_PTR(iseqw);
+    rb_iseq_t **iseq_ptr;
+    TypedData_Get_Struct(iseqw, rb_iseq_t *, &iseqw_data_type, iseq_ptr);
+    rb_iseq_t *iseq = *iseq_ptr;
 
     if (!ISEQ_BODY(iseq)) {
         rb_ibf_load_iseq_complete(iseq);