feat(frontends/lean/elaborator): generate type information for placeholders

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
perf(library/num): avoid exception

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/pp): pretty print numerals

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