ERA OF DOWNSIZING CREATES JOB BOOM FOR CLASS OF '97
College graduating class of 1997 is entering most auspicious job market in memory, product of soaring economy and large-scale layoffs of 1990's that cut swath through parents' generation; colleges, universities and professional schools report record numbers of recuiters on campus, often offering more jobs than they can fill; many experts believe corporations laid off too many employees in early 1990's and are recruiting many students now that business has boomed beyond expectations; photos (M)