feat(util/list_fn): add iter function
feat(kernel/expr): add expr::operator() which takes 8 args
refactor(library/rewriter/fo_match): add more lean_trace for debugging
feat(library/rewriter): implement repeat/app/lambda/pi/try rewriter

 - refactor to use rewriter_cell
 - implement display and operator<< for debugging
test(library/rewriter): add more tests