One rule over every allocating operation, so it has to hold for map-new,
put, reserve and clone exactly as it holds for vec-new, push, reserve
and clone. put stays Unit and clone stays the container; nothing grows a
Result.
A map is the harder of the two and that is why it gets its own program.
A Vec's failing allocation leaves the Vec untouched, whereas a map's
growth allocates a whole new block, rehashes into it and only then
releases the old one — so a failure partway has to leave the map exactly
as it was or the retry re-attempts against a half-moved map. 300 entries
through several grows against a ceiling that is raised each time, then
every one of them read back: no entry lost, none doubled.
