Attempt to mmap the input and output when compressing streams.

This short circuits the whole SquashBufferStream hack when possible,
which can be a huge performance boost (50-100% for several codecs).

Signed-off-by: Evan Nemerson <evan@coeus-group.com>
benchmark: improve benchmark presentation

This adds some more charts and improves some others.  Also included
are some minor build system improvements.

Signed-off-by: Evan Nemerson <evan@coeus-group.com>