Second function in `compare` reproducibly benchmarks faster than the first

With the use of `compare`, if identical functions are supplied, the second will reproducibly benchmark ~10-30% faster than the first, regardless of what the functions are.

SSCCE:
```
module Test exposing (main)

import Benchmark exposing (Benchmark)
import Benchmark.Runner exposing (BenchmarkProgram, program)

main : BenchmarkProgram
main =
program suite

suite : Benchmark
suite =
Benchmark.compare "First vs. Second" "First" (\_ -> myFunction "zxcvbn") "Second" (\_ -> myFunction "zxcvbn")

myFunction : String -> String
myFunction =
String.reverse
```
and some results for various functions in `myFunction`:

This behavior extends to more complex functions as well, just presenting simple ones here so as to eliminate variables.