feat(frontends/lean/parser_expr): 'namespaces' have precedence over 'using'

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): add unary_nat command, it forces Lean to convert numerals into unary encoding using nat::succ and nat::zero

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(tests/lean): adjust some tests affected by recent changes

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