Only apply 'tight' spacing to \mathbin cases

As details in issue #241, in text mode using tight spacing will
mess up minus signs here as a search-and-replace fails. Moreover, for
these cases at least there is no sense in applying tight spacing.

A couple of problems are outstanding:
 - the second example in the question when "output-exponent-marker"
   is set
 - the sign between a real and complex parts of a number, which
   is \mathbin but can be minus