Bench modules (ExUnit-style benchmarks definitions) (DSL)

I am thinking about writing support for ExUnit-style benchmarks (if there is no work toward such thing yet). And I am having something like that in mind:

```elixir
defmodule SampleBench do
use Benchee.Unit

bench do
input "Small (1 Thousand)", do: Enum.to_list(1..1_000)
input "Middle (100 Thousand)", do: Enum.to_list(1..100_000)

setup_all input do
{Enum.shuffle(input), fn i -> [i, i + 1] end}
end

func "flat_map", {input, func} do
Enum.flat_map(input, func)
end

func "map.flatten", {input, func} do
input |> Enum.map(func) |> List.flatten
end
end
end
```

Any suggestions or other propositions for the syntax?

The issue proposes an ExUnit-style benchmark DSL through a SampleBench module using Benchee.Unit, but names no implementation files or tests. First review the existing benchmark-definition API, then determine whether the proposed syntax and semantics have a clear design; done would mean an agreed DSL specification and corresponding implementation and tests.

Written by the indexing model from the issue text.