Some `unsafe` code in arrow is seemingly unsound or not very well documented.

Hi all,
while going through the code in `arrow`, I spotted a few places in which the usage of unsafe code is unsound, and a few in which I could not determine whether it was or not.

I do not believe that the issues I found could cause problems *today*, but I prepared a PR nonetheless that fixes the unsound cases I could find, improves documentation in the cases in which I could figure out why the code is indeed sound, and leaves a FIXME comment for the remaining cases that were beyond my ability to figure out :-)

I hope this helps!

Start by reviewing the unsafe code in arrow and the cases described in the issue. Determine which uses are unsound, document why the cases that are sound are safe, and leave FIXME comments for cases that cannot be established; done means these findings are addressed or explicitly recorded.

Written by the indexing model from the issue text.