Use atomics to count shared_object without locks

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C++11 (and subsequent C++ standards) provide portable ways to issue
atomic hardware instructions, which allow multiple threads to load,
store, and modify integers without taking a lock. The standard also
defines a memory model that lets you express the ordering guarantees
around these atomic operations. (x86 is relatively strongly-ordered, but
many other common architectures, such as ARM, are free to reorder loads
and stores unless told not to.)

This patch removes the lock from shared_object and replaces it with the
standard thread-safe reference counting implementation used in
C++'s std::shared_ptr, Rust's std::sync::Arc, and many others.

Additional resources on the topic:
https://assets.bitbashing.io/papers/concurrency-primer.pdf
https://www.youtube.com/watch?v=ZQFzMfHIxng

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