Improved throughput 4x by using mmap

... but abandoning this for now since it's complex and stdin handling is
problematic with this approach.

The throughput of the original implementation on my system was about
8MB/s, the throughput of this version is about 33MB/s. I suppose much of
that time is spent decoding multibyte sequences. Some more low-level
multibyte handling (as this tool really has no need of *parsing* them,
it just needs to recognize the boundaries between) would be useful.