refactor(library/standard): use new coding style, rename bool.b0 and bool.b1 to bool.ff and bool.tt

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/structure): add 'structure' command skeleton

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard/list.lean): begin to port lists from lean 0.1