refactor(library/sigma): fix/use sigma notation
feat(frontends/lean/pp): add option 'pp.numerals'
fix(frontends/lean/token_table): increase binding power of decreasing operator
feat(library/util): add auxiliary functions for creating tuples (using sigma types)
feat(kernel/type_checker): add ensure_type variant
feat(frontends/lean): type check 'decreasing' proofs in definition using well-founded recursion
feat(library/init/logic): add is_true and is_false
feat(util/buffer): add insert method
feat(frontends/lean/elaborator): replace metavariables in the equation lhs with fresh local constants before invoking compiler
fix(tests/lean/interactive): adjust test to reflect changes in the standard library