Added option to manually specify a reference to compare the results to.

[JanEricNitschke]

I was interested in the functionality discussed in #579 and #577.

Sadly the former PR has been stale for over a year.

So i have tried to effectively take the work from there, make it fit with the changes that have happend since then and also address the comments in the PR.

There are currently still two points that i am myself unsure about.

I have used the standard std::cmp::Ordering for the ordering compared to the reference. However, i feel that the naming is not actually clear at a first glance. Namely that Ordering::Less corresponds to a faster time (the runtime is less). It might be worth to create a distinct enum that maps to this (converts from) as suggested in the original PR review.

I am also not sure if the statements faster than and slower than are the best. Particularly the latter feels a bit unintuitive to me.
Maybe something like 0.2 times as fast would be better than 5.0 times slower?