[Deque] append(contentsOf:): Use exponential capacity reservation

[lorentey]

Depending on the supplied sequence, Deque.append(contentsOf:) currently reserves just enough capacity as it needs to perform the append. This isn't right -- it can result in O(n) reallocs instead of O(log(n)) when items are incrementally appended in constant-sized append(contents:) batches.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[lorentey]

@swift-ci test

[lorentey]

We'll need to figure out how to test this sort of allocation behavior. The obvious thing is to add some counters to the storage buffer in SWIFT_COLLECTIONS_INTERNAL_CHECKS builds.

[lorentey]

@swift-ci test