refactor(kernel/expr): remove the dangerous expr::release method

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(library/tactic): remove 'null' tactic, and operator bool tactics

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/parser): reachable code

The new test nbug1.lean exposes the problem.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(kernel/formatter): hide 'unsafe' constructor

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