A Teachers' Pet Is Facing the Hounds
Article on financial strategy of John H Biggs, chairman of Teachers Insurance and Annuity Association-College Retirement Equities Fund, world's biggest pension system; focuses on Biggs's opposition to provision in tax bill, now in Congress, to strip his organization of its tax-exempt status; says Biggs is fighting provision even though the change would give his pension fund the opportunity to branch out into new areas; charts; photos (L)