Improve performance of Integer#ceildiv

This patch is suggested by nobu.

Benchmark result:

```
require 'benchmark'

n = 10 ** 7

Benchmark.bm do |x|
  x.report("Fixnum/Fixnum") { a, b = 5, 2; n.times { a.ceildiv(b) } }
  x.report("Bignum/Bignum") { a, b = 10**100, 10**99 - 1; n.times { a.ceildiv(b) } }
  x.report("Bignum/Fixnum") { a, b = 10**100, 3; n.times { a.ceildiv(b) } }
end
```

Original:

```
       user     system      total        real
Fixnum/Fixnum  3.340009   0.043029   3.383038 (  3.384022)
Bignum/Bignum  8.229500   0.118543   8.348043 (  8.349574)
Bignum/Fixnum  8.328971   0.097842   8.426813 (  8.426952)
```

Improved:

```
       user     system      total        real
Fixnum/Fixnum  0.699140   0.000961   0.700101 (  0.700199)
Bignum/Bignum  5.076165   0.083160   5.159325 (  5.159360)
Bignum/Fixnum  5.548684   0.115372   5.664056 (  5.666735)
```

1 files changed, 17 insertions, 0 deletions

diff --git a/numeric.rb b/numeric.rb
index 9f2200d2a8..c2091465f8 100644
--- a/numeric.rb
+++ b/numeric.rb
@@ -227,6 +227,23 @@ class Integer
     Primitive.attr! 'inline'
     Primitive.cexpr! 'rb_int_zero_p(self)'
   end
+
+  #  call-seq:
+  #    ceildiv(other) -> integer
+  #
+  #  Returns the result of division +self+ by +other+. The result is rounded up to the nearest integer.
+  #
+  #    3.ceildiv(3) # => 1
+  #    4.ceildiv(3) # => 2
+  #
+  #    4.ceildiv(-3) # => -1
+  #    -4.ceildiv(3) # => -1
+  #    -4.ceildiv(-3) # => 2
+  #
+  #    3.ceildiv(1.2) # => 3
+  def ceildiv(other)
+    -div(-other)
+  end
 end
 
 #  call-seq: