Turn simple-bench into a proper benchmark

[RyanGlScott]

simple-bench is essentially a benchmark, so why not label it as such? This:

• Turns simple-bench from an executable to a benchmark, making it possible to enable with just --enable-benchmarks (I had to turn on the bench Cabal flag by default to accomplish this).
• Fixes build warnings in simple-bench. While doing this, I discovered that we don't actually need to depend on mtl in simple-bench, so I removed that dependency.
• Tweaks .travis.yml to compile the benchmarks on each run.

[erikd]

Thanks Ryan.

I've just taken over maintainer ship of this package and these things were on my TODO list. Not they are not :).