TeX writer:  Removed need for fixTree.

Handle accents directly in Under/Over clauses.
(reorderDiacriticals also needed to go because we're removing
EUnary.)

AS a side effect, we have somewhat better output for
over/under accents.
Removed EUnary, added ESqrt and ERoot.

This is cleaner; we don't need to pass around TeX command
names in the AST.