Assertion memoization for `proveTrue` inside `Option::do`

[klimick]

It would be nice if proveTrue can memoize assertions:

/**
 * @return Option<positive-int>
 */
function provePositiveInt(?int $number): Option
{
     return Option::do(function() use ($number) {
         yield proveTrue(null !== $number);
         yield proveTrue($number > 0);

         // for now it will be type error, because assertions from above will be ignored
         return $number;
     });
}