box: support computer modern brackets and radicals

This patch modifies neateqn to use computer modern glyphs for
brackets and radicals, if they are available.  Now, neateqn
first tries to find the smallest bracket or radical that
can contain the inner box; otherwise, it builds the bracket
using bracket pieces.  This patch also modifies box_beforeput()
to perform italic correction if necessary.  To make the brackets
pack better, especially for computer modern, tok_len() decreases
the height and depth of its argument.