Array natives

This adds `Array`-specific implementations of some `Enumerable` methods. In some cases (see notes below), two methods are synonymous, but `Array` only overrode one of them. I focussed on `Enumerable` methods that we actually use in Shopify. There are some more esoteric methods like `chunk`, `minmax`, `slice_before` and `slice_after` that I ignored.

I've implemented this against a recent MRI `trunk`. It should be a trivial backport to https://github.com/Shopify/ruby.

r: @camilo @csfrancis @jasonhl
cc: @grollest @sirupsen

### Without change:
```
                       user     system      total        real
all? {}            0.660000   0.000000   0.660000 (  0.663685)
all?               0.150000   0.000000   0.150000 (  0.150084)
include?           0.330000   0.010000   0.340000 (  0.340451)
member?            0.510000   0.000000   0.510000 (  0.508890)
one? {}            0.630000   0.000000   0.630000 (  0.632244)
one?               0.150000   0.010000   0.160000 (  0.156780)
none?              0.150000   0.000000   0.150000 (  0.156365)
none? {}           0.740000   0.000000   0.740000 (  0.744607)
detect             0.560000   0.000000   0.560000 (  0.563873)
find               0.550000   0.000000   0.550000 (  0.561899)
each_with_index    1.010000   0.000000   1.010000 (  1.009375)
each_with_object   0.590000   0.010000   0.600000 (  0.588555)
partition          0.920000   0.040000   0.960000 (  0.966469)
grep               1.950000   0.000000   1.950000 (  1.943385)
grep {}            1.890000   0.000000   1.890000 (  1.898237)
flat_map           1.470000   0.070000   1.540000 (  1.539217)
collect_concat     1.530000   0.040000   1.570000 (  1.571326)
to_a               0.000000   0.000000   0.000000 (  0.000008)
entries            0.300000   0.000000   0.300000 (  0.306828)
select             0.560000   0.040000   0.600000 (  0.611437)
find_all           0.730000   0.000000   0.730000 (  0.742103)
```

### With change:
```
                       user     system      total        real  improvement
all? {}            0.430000   0.000000   0.430000 (  0.434470)      1.53
all?               0.010000   0.000000   0.010000 (  0.007103)     21.13
include?           0.330000   0.010000   0.340000 (  0.339223)      1.00
member?            0.340000   0.000000   0.340000 (  0.340663)      1.49
one? {}            0.420000   0.010000   0.430000 (  0.425346)      1.49
one?               0.010000   0.000000   0.010000 (  0.010269)     15.27
none?              0.010000   0.000000   0.010000 (  0.010471)     14.93
none? {}           0.470000   0.000000   0.470000 (  0.472409)      1.58
detect             0.400000   0.010000   0.410000 (  0.413766)      1.36
find               0.440000   0.000000   0.440000 (  0.445540)      1.26
each_with_index    0.800000   0.000000   0.800000 (  0.802467)      1.26
each_with_object   0.420000   0.010000   0.430000 (  0.426938)      1.38
partition          0.590000   0.040000   0.630000 (  0.634688)      1.52
grep               1.600000   0.010000   1.610000 (  1.612733)      1.21
grep {}            1.690000   0.010000   1.700000 (  1.701325)      1.12
flat_map           1.380000   0.080000   1.460000 (  1.461663)      1.05
collect_concat     1.560000   0.060000   1.620000 (  1.611663)      0.97
to_a               0.000000   0.000000   0.000000 (  0.000010)      0.80
entries            0.000000   0.000000   0.000000 (  0.000010)  30682.80
select             0.580000   0.000000   0.580000 (  0.574380)      1.06
find_all           0.560000   0.010000   0.570000 (  0.566607)      1.31
```

### Notes:
- `include?` and `member?` are synonyms, but `member?` was calling the `Enumerable` version of the method. The `Array` version is about 50% faster.
- `flat_map` and `collect_concat` are synonyms. There doesn't seem to be much benefit to a custom `Array` implementation.
- `to_a` and `entries` are synonyms, but `entries` was calling the `Enumerable` version of the method. The `Array` version is a little faster.
- `select` and `find_all` are synonyms, but `find_all` was calling the `Enumerable` version of the method. The `Array` version is about 30% faster.
- my implementation of `inject`/`reduce` is buggy, so I've disabled it for now. Given the complexity of the code, I'm not convinced there's a huge win here, other than removing a `memo` object allocation.
- some of these methods remove a `memo` object allocation - I'll add a few benchmarks with empty arrays to measure the benefit of removing those allocations.
- the `grep` benchmark is a bit shady because the block is only called for matching elements, and the content of `array` is random.

### Benchmarks:
```Ruby
require 'benchmark'

array = (1..9999999).map { rand }
array << 1.0

true_array = (1..10000000).map { true }
false_array = (1..10000000).map { false }
mostly_false_array = false_array.dup
mostly_false_array[mostly_false_array.length-1] = true

Benchmark.bmbm do |x|
  x.report("all? {}") { array.dup.all? {|e| e >= 0.0 } }
  x.report("all?") { true_array.dup.all? }
  x.report("include?") { array.dup.include?(1.0) }
  x.report("member?")  { array.dup.member?(1.0) }
  x.report("one? {}") { array.dup.one? {|e| e == 1.0 } }
  x.report("one?") { mostly_false_array.dup.one? }
  x.report("none?") { false_array.dup.none? }
  x.report("none? {}") { array.dup.none? {|e| e < 0.0 } }
  x.report("detect") { array.dup.detect {|e| e == 1.0 } }
  x.report("find") { array.dup.find {|e| e == 1.0 } }
  x.report("each_with_index") { s = 0; array.dup.each_with_index {|e, i| s += e+i }}
  x.report("each_with_object") { array.dup.each_with_object(0) {|e, o| }}
  x.report("partition") { array.dup.partition {|e| e < 0.5 }}
  x.report("grep") { array.dup.grep 0.4...0.5 }
  x.report("grep {}") { s = 0; array.dup.grep(0.4...0.5) {|e| s += e } }
  x.report("flat_map") { array.dup.flat_map {|e| [e, -e] } }
  x.report("collect_concat") { array.dup.collect_concat {|e| [e, -e] } }
  x.report("to_a") { array.dup.to_a }
  x.report("entries") { array.dup.entries }
  x.report("select") { array.dup.select {|e| e < 0.5 } }
  x.report("find_all") { array.dup.find_all {|e| e < 0.5 } }
end
```
Why does this need to be `volatile`?