Optimize Enum.count/2

[michalmuskala]

reduce/3 has clauses specialized for various enumerable types, so should be
much faster than the general Enumerable.reduce/3 case, especially that we don't
need additional facilities like terminating the reduction early.

Comparing 3 implementations:

• map based

  elements |> Enum.map(&(if check(&1), do: 1, else: 0)) |> Enum.sum
  

• count/2 based

  elements |> Enum.count(&check/1)
  

• filter based

  elements |> Enum.filter(&check/1) |> Enum.count
  

We get following results before and after the change:

len	map	count before	count after	filter
10	2.1706 μs	7.0754 μs	1.6304 μs	1.9765 μs
100	19.921 μs	27.579 μs	13.057 μs	14.591 μs
1000	168.93 μs	178.93 μs	130.76 μs	163.43 μs
10000	2128.9 μs	1822.1 μs	1298.2 μs	1664.6 μs

The most idiomatic code is now also the fastest.