refactor(frontends/lean): remove notation for creating tuples

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/elaborator): bug reported by Jeremy Avigad

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(builtin/num): remove hacks for making the elaborator happy

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>