Revert "add_heap_page should not be invoked in gc_alloc, but should be in obj_alloc"

This reverts commit f11623f07554a470e5f9855459c9aefa3f48b48a.
Revert "pluggable heap pages"

This reverts commit e6523730a45bbc0ccc84c21c3edb7cc3ac5e867a.