Simplify hash table drops

Simplify hash table drops

This replaces the `std::collections::hash::table::RevMoveBuckets`
iterator with a simpler `while` loop.  This iterator was only used for
dropping the remaining elements of a `RawTable`, so instead we can just
loop through directly and drop them in place.

This should be functionally equivalent to the former code, but a little
easier to read.  I was hoping it might have some performance benefit
too, but it seems the optimizer was already good enough to see through
the iterator -- the generated code is nearly the same.  Maybe it will
still help if an element type has more complicated drop code.