`result` value replacement operator

E.g. `r1 & r2`. The result

* contains `r1.error()` if `r1.has_error()`, or
* contains `r2.error()` if `r2.has_error()`, or
* contains `r2.value()` otherwise.

As you mentioned on Slack, maybe this should only be allowed when `r1` is `result<void>`, although I personally don't see a problem with it.