Faster array spec

This speeds up the array spec by about 2 seconds. I'm sure lots of other specs have similar optimizations.

```
farleyknight@farleyknight-desktop:~/dev/crystal/crystal$ rspec spec/codegen/faster_array_spec.rb
Run options: exclude {:primitives=>true, :integration=>true}
.........

Finished in 3.81 seconds
9 examples, 0 failures
farleyknight@farleyknight-desktop:~/dev/crystal/crystal$ rspec spec/codegen/faster_array_spec.rb 
Run options: exclude {:primitives=>true, :integration=>true}
.........

Finished in 3.48 seconds
9 examples, 0 failures
farleyknight@farleyknight-desktop:~/dev/crystal/crystal$ rspec spec/codegen/array_spec.rb 
Run options: exclude {:primitives=>true, :integration=>true}
.........

Finished in 5.67 seconds
9 examples, 0 failures
farleyknight@farleyknight-desktop:~/dev/crystal/crystal$ rspec spec/codegen/array_spec.rb 
Run options: exclude {:primitives=>true, :integration=>true}
.........

Finished in 5.57 seconds
9 examples, 0 failures
```