Fix super/subscript issue.

We were getting the following XeLaTeX error:

! LaTeX Error: Command \textsubscript unavailable in encoding EU1.

for the input "(C$_6$H$_10$O$_5$)\textsubscript{n}" in the papier.zip
test case.  This seems to be a fight between the xunicode and fontspec
packages, sigh.  Work around this by using "real" super/subscripts,
from the xltxtra package.  However, the "real" superscript doesn't handle
the [] characters, used in references.  So use \textsuperscript* to
fall back to the "old" superscript implementation.

While we're at it, fix the issue (revealed by the input above) where
we weren't properly brace-enclosing sub/superscripts of more than one
character.

Change-Id: I7b08d74a215312e2cb6ab1e432b94c920d5fd85f