Extract atomic-u64 crate

[kevinheavey]

Problem

solana_program::atomic_u64 blocks moving a bunch of modules out of solana-program

Summary of Changes

Move atomic_u64 to its own crate and put "For internal use only" in the description since it was not a public module

[buffalojoec]

Nice! Looks good to me, and we don't even have to worry about deprecation, since it was all pub(crate).

Is there any reason why we may introduce other atomic integer values? Should we not shackle ourselves here and maybe call this solana-atomic-integer? What do you think?

[kevinheavey]

Meh, crates are free and the only constraint this imposes is that someone who wants to do something different in the future will have to make a new crate instead of bloating this one

[buffalojoec]

Meh, crates are free and the only constraint this imposes is that someone who wants to do something different in the future will have to make a new crate instead of bloating this one

Fair enough, but the word "bloating" is a stretch for a crate that's 38 lines long.

Just give this a quick rebase and I'll stamp it. Thanks!