Add notice about memcpy.
Add `compressed` bit and `last block offset` to stream header.

The compressed_stream no longer uses user data to store the header stuff
that is specific to the compressed_stream.

stream_accessor::open now takes an additional parameter,
`preferCompression`, and has get/set_compressed to figure out if the
stream is actually using compression.
Support reading uncompressed streams with compressed_stream.
Add ut-compressed_stream uncompressed.
Change compressed_stream to support uncompressed overwrites interleaved with reads.
Extend ut-compressed_stream uncompressed test to test writes.
Update documentation to reflect support for uncompressed streams.
Add compressionFlags to open, to support creating an uncompressed stream with compressed_stream.
Parameterise ut-compressed_stream tests on the compression flags, and add versions where compression is disabled.