Switch indentation from spaces to tabs.

Switch indentation from spaces to tabs.

For the longest time I had a fairly strong personal preference for
spaces.  Then it came to my attention that using tabs meaningfully
increases accessibility.

As a cryptography library, Monocypher is supposed to ultimately help,
among other people, the most vulnerable among us.  It would be a shame
to potentially exclude disabled contributors or auditors.

Note that this patches sometimes changes a little more than just
spacing.  A few pieces of code in particular relied on indentation
width, and had to be reworked a little bit to make them tab width
agnostic.