It's unused.

It's only used once, for `VecDeque`, and can easily be replaced by
something else. The commit changes `grow_if_necessary` to `grow` to
avoid some small regressions caused by changed inlining.

The commit also removes `Strategy::Double`, and streamlines the
remaining variants of `Strategy`.

It's a compile time win on some benchmarks because the many
instantations of `RawVec::grow` are a little smaller.

The amortized case is much more common than the exact case, and it is
typically instantiated many times.

Also, we can put a chunk of the code into a function that isn't generic
over T, which reduces the amount of LLVM IR generated quite a lot,
improving compile times.