Before this patch `reserve` function allocated twice as requested
amount elements (not twice as capacity). It leaded to unnecessary
excessive memory usage in scenarios like this:
```
let mut v = Vec::new();
v.push(17);
v.extend(0..10);
println!("{}", v.capacity());
```
`Vec` allocated 22 elements, while it could allocate just 11.
`reserve` function must have a property of keeping `push` operation
cost (which calls `reserve`) `O(1)`. To achieve this `reserve` must
exponentialy grow its capacity when it does reallocation.
There's better strategy to implement `reserve`:
```
let new_capacity = max(current_capacity * 2, requested_capacity);
```
This strategy still guarantees that capacity grows at `O(1)` with
`reserve`, and fixes the issue with `extend`.
Patch imlpements this strategy.
I needed it in `RawVec`, `Vec`, and `TypedArena` for `rustc` to
bootstrap; but of course that alone was not sufficient for `make
check`.
Later I added `unsafe_destructor_blind_to_params` to collections, in
particular `LinkedList` and `RawTable` (the backing representation for
`HashMap` and `HashSet`), to get the regression tests exercising
cyclic structure from PR #27185 building.
----
Note that the feature is `dropck_parametricity` (which is not the same
as the attribute's name). We will almost certainly vary our strategy
here in the future, so it makes some sense to have a not-as-ugly name
for the feature gate. (The attribute name was deliberately selected to
be ugly looking.)
