sort(1): Memoize MD5 computation to reduce repeated computation

Experimentally, reduces sort -R time of a 148160 line corpus from about
3.15s to about 0.93s on this particular system.

There's probably room for improvement using some digest other than md5, but
I don't want to look at sort(1) anymore. Some discussion of other possible
improvements in the Test Plan section of the Differential.

PR: 230792
Reviewed by: jhb (earlier version)
Differential Revision: https://reviews.freebsd.org/D19885