Compressed shared_ptr
handle is a 64-bit integer where:
- Lower 42 bits represent the pointer bits.
- Upper 22 bits represent the reference count.
// +----------------------+----------------------+
// | 22 bits refcount | 42 bits pointer |
// +----------------------+----------------------+
//
// bit layout example:
// [ 63 .................... 42 | 41 ................................ 0 ]
// [ refCount | rawPtr ]