openpgp: Improve the streaming Decryptor's buffer strategy.

  - Previously, for a read of X bytes, we'd request X + buffer_size
    from the underlying buffered reader, then satisfy the read
    request, after which we'd request the next X + buffer_size bytes
    for the next read.  This requires the underlying reader to copy
    buffer_size bytes for each read.  In a typical scenario, we'd copy
    25 megabytes (the default buffer size) for every 8 kilobytes
    read (std::io::copy's default buffer size).  This incurs a linear
    cost with a very high factor.

  - Improve this by requesting 2 * buffer_size, then satisfying the
    reads from the first half of that buffer, only consuming the first
    half once we have exhausted the first half.  Then, we'd request
    the next 2 * buffer_size, at which point the underlying buffered
    reader has to copy the data to a new buffer.

  - See #771.

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