[Deque/HashTree] Remove references to ManagedBuffer.capacity and use the given minCapacity

[Azoy]

When creating a ManagedBuffer object, the factory closure passes the newly created ManagedBuffer instance for the caller to make the header with. Referencing ManagedBuffer.capacity will end up making calls to malloc_size to return the allocation size, but the caller has already supplied it with a minimumCapacity argument. For embedded, it's critical that we don't needlessly link against system symbols that may or may not exist especially since the information we need is already there (and in fact the code was already guarding against it for OpenBSD).

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]

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