Add Enum.reduce/2 that sets initial value of acc to the first element of collection

Example:

```elixir
1..3 |> Enum.reduce(&1 + &2)
```
This is similar to Scala:

```scala
(1 to 3).reduce(_ * _)
```

And F#:
```fsharp
[1..3] |> List.reduce(*)
```