Range#each fixnums are special

Range#each is commonly-used.
And Integer#succ behavior is ensured by ISO.

So, I propose that avoid check `respond_to?(:succ)` and call `Fixnum#+` method if Range#each both arguments are `Fixnum`

---

for reference

```
$ uname -a
Linux localhost.localdomain 2.6.32-279.el6.x86_64 #1 SMP Fri Jun 22 12:19:21 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux
```

```
t = Time.now

(0..1000000).each do
end

puts Time.now - t
```

before

```
0.34153699999999
```

after

```
0.16073899999999
```