Avoid allocating the second array by using `Array#reject!` instead of
`Enumerable#partition` in `Array#extract!`.

There are benchmarks in order to ensure that the changes speed up the method:
```
begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update
your Bundler"
  raise e
end

class Array
  def extract_v1!(&block)
    unless block_given?
      to_enum(:extract!) { size }
    else
      extracted_elements, other_elements = partition(&block)

      replace(other_elements)

      extracted_elements
    end
  end

  def extract_v2!
    return to_enum(:extract!) { size } unless block_given?

    extracted_elements = []

    reject! do |element|
      extracted_elements << element if yield(element)
    end

    extracted_elements
  end
end

gemfile(true) do
  source "https://rubygems.org"

  gem "benchmark-ips"
end

arrays_for_partition = Array.new(1000) { (0..10000).to_a }
arrays_for_extract_v1 = Array.new(1000) { (0..10000).to_a }
arrays_for_extract_v2 = Array.new(1000) { (0..10000).to_a }

Benchmark.ips do |x|
  x.report("Array#partition")  do
    arrays_for_partition.each do |numbers|
      odd_numbers, numbers = numbers.partition { |number| number.odd? }
      numbers
    end
  end

  x.report("Array#extract_v1!")  do
    arrays_for_extract_v1.each do |numbers|
      odd_numbers = numbers.extract_v1! { |number| number.odd? }
      numbers
    end
  end

  x.report("Array#extract_v2!")  do
    arrays_for_extract_v2.each do |numbers|
      odd_numbers = numbers.extract_v2! { |number| number.odd? }
      numbers
    end
  end

  x.compare!
end
```

The result of the benchmarks:

```
ruby -v
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
```

```
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Using benchmark-ips 2.7.2
Using bundler 1.16.1
Warming up --------------------------------------
     Array#partition     1.000  i/100ms
   Array#extract_v1!     1.000  i/100ms
   Array#extract_v2!     1.000  i/100ms
Calculating -------------------------------------
     Array#partition      1.390  (± 0.0%) i/s -      7.000  in   5.044843s
   Array#extract_v1!      2.781  (± 0.0%) i/s -     14.000  in   5.050589s
   Array#extract_v2!      3.151  (± 0.0%) i/s -     16.000  in   5.080608s

Comparison:
   Array#extract_v2!:        3.2 i/s
   Array#extract_v1!:        2.8 i/s - 1.13x  slower
     Array#partition:        1.4 i/s - 2.27x  slower
```

Avoid `unless`/`else` in favour of an early return.
The double-negative of that `else` can be confusing,
even though the code layout is nearly the same.
Also using of early return would improve `git diff`
if we needed to change this method.