Replace \begin{spacing} with \setstretch{} calls.

Replace \begin{spacing} with \setstretch{} calls.

setspace's spacing environment adds some extra skips to get
clean transitions.  We handle those transitions on our own.
\setstretch lets us change the spacing without bothering
with the extra skipping.

I've been struggling with \blanklines.  Current problems:
  * page top--chapter head space longer than one line.
  * chapter head--text space longer than chapter head--section head.
  * head--table space longer for LOT/LOF than TOC.