benchmark

Custom user counters: add invert modifier. (#850)

While current counters can e.g. answer the question
"how many items is processed per second", it is impossible to get
it to tell "how many seconds it takes to process a single item".

The solution is to add a yet another modifier `kInvert`,
that is *always* considered last, which simply inverts the answer.

Fixes #781, #830, #848.