* include/ruby/ruby.h (RARRAY_LEN):  change more faster get embed array length

i try benchmark this change.

```ruby
require 'benchmark'

n = 10000000

Benchmark.bm do |x|
  GC.start
  empty_time = x.report("empty"){n.times{ }}
  GC.start
  embed_time = x.report("embed length"){n.times{ [].length }}
  puts embed_time.real / empty_time.real
end
```

result of 10 times average.

before: 1.681396211600276
after: 1.6629606856866217

it's more faster but just a little :ant: