Always comment on every `unsafe` usage describing why the `unsafe` usage is sound

[BarryBo]

    It would be really awesome if this library always had comments on every `unsafe` usage describing why the `unsafe` usage is sound. Something like:

// UNSAFE: $REASON

Originally posted by @rylev in #101 (comment)

[BarryBo]

I filed a work item for this. The blanket answer is they are all calling through FFI code with types that are unsafe. But I'll make a pass and comment them all, so lint tools are happy :-)