Make blockvector a little more self-contained

Make blockvector a little more self-contained

This patch changes blockvector to be a little more self-contained.

The idea here is that code outside of blockvector shouldn't really
know how it operates.  After this patch, this still doesn't fully
happen -- a couple spots check the result of map() and make decisions
based on that -- but this is a step toward making that happen.  The
longer term idea here is that this is needed to enable lazier CU
expansion.

Meanwhile, this patch seems like a simple cleanup.  Relocation is now
handled by the blockvector itself and the non-const map() method can
be removed.

There wasn't a great spot to move the section_offsets typedef.  I
chose defs.h.  I've also updated the comment there as it has been out
of date for a long time.  I've also removed an obsolete comment from
the symbol-relocation code.

Regression tested on x86-64 Fedora 40.

Approved-By: Simon Marchi <simon.marchi@efficios.com>