Refactor and document ASSUME()

Refactor and document ASSUME()

The blead definition doesn't compile when I use Devel::PPPort earlier than
some point in the 5.19 series.

Examining the code, I noticed that there was a potential cyclic
definition: ASSUME can call NOTREACHED, and NOTREACHED can call assume.

Blead also assumed without checking that __builtin_unreachable() exists.

So, I refactored things so that there is no cycle, and changed a few
words in the comments to turn them into pod, and the new version
compiles all the way back.