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

Example:

```elixir
1..3 |> Enum.reduce(&1 + &2)  #=> 6
```
Similar in other langs:

Scala:
```scala
(1 to 3).reduce(_ * _)
```

F#:
```fsharp
[1..3] |> List.reduce(*)
```

Haskell:
```haskell
foldl1 (*) [1, 2, 3]
```

Ruby:
```ruby
(1..3).reduce(:*)
```