math expressions now have upright roman text

where it is requested by \rm, \mathrm, \log, \sin, \cos, etc.
Previously there was italic text everywhere except in \mbox
(newtxmath and unicode-math packages were interfering with font selection).

Plus removed some negative space between math symbols and made
division slashes smaller. These tweaks looked better with Libertine
font but are unnecessary with Computer Modern.
Applyed patches given by Raba-san. Thank you, Raba-san!
Merge branch 'japanese' of https://github.com/minghai/sicp-pdf into Japanese
Applyed patches given by Nona-san. (@nna774) Thanks!
Applyed patches given by Nona-san and Raba-san. Thanks!