Rework tor_bytes::Truncated

Fixes #1614 (closed)

Roughly along these lines #1614 (comment 3074562)

• I tried const generics and it's quite syntactic vinegar. In my experiments turning Reader into a trait looked like it would help but isn't entirely straightforward and also probably not uncontroversial.
• I used from_possibly_incomplete_slice for the new constructor. There are 8 call sites all in the same formulaic code in tor-socksproto, which is probably ripe for tidying up anyway (#1590). FTR I don't mind renaming it (in this MR if we can come up with a good name quickly).
• I did rename Truncated. That also removes what was an API break.
• I found it best to have test cases use the from_possibly_incomplete_slice Reader, but calling that in test cases was odd, so I invented Reader::from_slice_for_test.
• I renamed a whole bunch of Reader variables from r to b since I kept tripping over them while doing bulk changes