benchmark: add getrusage support, separate timer into its own file

OS X doesn't support clock_gettime, so if it is not available we fall
back on getrusage.  This also paves the way for adding Windows support
(via GetProcessTimes).

https://github.com/quixdb/squash/issues/49

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