print debug junk to stderr, not stdout

Change-Id: Ieb47c6bb3597813b7298c9208041c1a80d8e0916
Reviewed-on: https://gerrit.spotify.net/gerrit/65432
TC-Compile-And-Test: Team City <teamcity@spotify.com>
Reviewed-by: Erik Bernhardsson <erikbern@spotify.com>
Use mmap with MAP_POPULATE if available

This seems to help performance significantly, at least for the first
several keys if not for all of them.  With this change the variance in
lookup speed is very low.

Change-Id: Ie3a4508af54b3e65cf7bb1a14d7d490bacfaadc2
Reviewed-on: https://gerrit.spotify.net/gerrit/65949
Reviewed-by: Erik Bernhardsson <erikbern@spotify.com>
added picture to readme