Data.List: improve foldr

現行のやり方より `reverse(copy(a:xs))` を `foldl` に渡す方が速いようです。

ループ部分のベンチマーク
https://gist.github.com/ichizok/6914387#file-fold-test-vim

OS X 10.8.5 / Core i5 2.5GHz / 16GB mem

```
s:foldr_orig
----------------
[ 1]   0.006166
[ 2]   0.006257
[ 3]   0.006136
[ 4]   0.006181
[ 5]   0.006225
[ 6]   0.006158
[ 7]   0.006165
[ 8]   0.006176
[ 9]   0.006175
[10]   0.006254
----------------
total: 0.061893, avg: 0.006189
================================

s:foldr_invr
----------------
[ 1]   0.006251
[ 2]   0.006171
[ 3]   0.006182
[ 4]   0.006159
[ 5]   0.006187
[ 6]   0.006168
[ 7]   0.006162
[ 8]   0.006135
[ 9]   0.006197
[10]   0.006147
----------------
total: 0.061759, avg: 0.006176
================================

s:foldr_copy
----------------
[ 1]   0.005405
[ 2]   0.005400
[ 3]   0.005351
[ 4]   0.005382
[ 5]   0.005363
[ 6]   0.005391
[ 7]   0.005388
[ 8]   0.005383
[ 9]   0.005352
[10]   0.005391
----------------
total: 0.053806, avg: 0.005381
================================
```