Taylor Branch article on 36-yr-old Saul Kripke, considered by colleagues as 'one true genius' of philosophy; Kripke is described as prodigy of math and logic, as well as a cautious, tentative revolutionary in world of modern philosophy; his contributions to philosophy and plans for future research noted; extracts from Aristotle's Metaphysics, and Ludwig Wittgenstein's Tractatus Logico-Philosophicus; glossary of philosophical terms; illus (L)