Atomic immutable value for Rust.
$ cargo +nightly bench
running 6 tests
test multi_thread_load ... bench: 576 ns/iter (+/- 510)
test multi_thread_load_std ... bench: 1,113 ns/iter (+/- 1,130)
test multi_thread_store_and_load ... bench: 483 ns/iter (+/- 74)
test multi_thread_store_and_load_std ... bench: 27,897 ns/iter (+/- 6,171)
test single_thread_load ... bench: 22 ns/iter (+/- 1)
test single_thread_load_std ... bench: 41 ns/iter (+/- 0)