Use rb_hash_foreach in coverage

Using RHASH_TBL_RAW is a private API, so we should use rb_hash_foreach
rather than RHASH_TBL_RAW with st_foreach.

ext/coverage/coverage.c

@@ -353,7 +353,8 @@ rb_coverage_peek_result(VALUE klass)
         rb_raise(rb_eRuntimeError, "coverage measurement is not enabled");
     }
     OBJ_WB_UNPROTECT(coverages);
-    st_foreach(RHASH_TBL_RAW(coverages), coverage_peek_result_i, ncoverages);
+
+    rb_hash_foreach(coverages, coverage_peek_result_i, ncoverages);
 
     if (current_mode & COVERAGE_TARGET_METHODS) {
         rb_objspace_each_objects(method_coverage_i, &ncoverages);