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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.