Remove unsafe for some safe functions

[nw0]

Closes #1389, which noted that some safe functions are marked unsafe.

Noted that this is a breaking change.

[programmerjake]

iirc, removing unsafe shouldn't actually be a breaking change. all functions can be called in an unsafe block and also can be coerced to unsafe function pointers, so the worst that can happen is code gets a warning that it has a unnecessary unsafe block.

[davidhewitt]

I did say in #1389 that this would be breaking, but actually I think @programmerjake is correct. It's the same level of breaking as if we added #[deprecated] - it won't break builds automatically, only for those who #[deny] certain warnings.

Thanks for following up the discussion with this PR!