macOS: Try getentropy() then fallback to /dev/random

[josephlr]

Fixes #38. As we moved files around macos.rs -> ios.rs the diff for this PR is weird. However, the diffs for each individual commit are fine, so review will be easier if you look at those.

As /dev/random and /dev/urandom are the same on macOS. We just use /dev/random to keep our use_file implementation simple.

Tested on macOS with and without getentropy(2)

[newpavlov]

Can you also update lib.rs docs?

[josephlr]

Can you also update lib.rs docs?

Done, I had to link to unix.com's man pages as there doesn't seem to be a good way to view macOS manpages on Apple's website.

[newpavlov]

Great! Thank you!