Remove peek()

Removing peek() function to prepare for further optimization.

Performance is not changed.
Before
----
|Source|Size (KiB)|Time (ms, optimization disabled)|Time (ms)                                    |
|:---------:|---------:|--------:|----:|
|    Underscore 1.5.2|  42.5 KiB|  11.72 ms ± 0.81%|  5.43 ms ± 1.11%|
|      Backbone 1.1.0|  58.7 KiB|  13.48 ms ± 0.38%|  5.74 ms ± 0.98%|
|      MooTools 1.4.5| 156.7 KiB|  76.87 ms ± 3.74%| 45.63 ms ± 3.67%|
|        jQuery 1.9.1| 262.1 KiB| 101.20 ms ± 1.94%| 61.53 ms ± 2.54%|
|          YUI 3.12.0| 330.4 KiB|  85.74 ms ± 1.51%| 49.17 ms ± 1.27%|
| jQuery.Mobile 1.4.2| 442.2 KiB| 172.78 ms ± 2.08%|108.07 ms ± 3.23%|
|       Angular 1.2.5| 701.7 KiB| 148.74 ms ± 2.01%| 84.47 ms ± 2.35%|
|Total|1994.3 KiB|610.54|360.04|

After
----
|Source|Size (KiB)|Time (ms, optimization disabled)|Time (ms)|
|:---:|---:|---:|---:|
|Underscore 1.5.2|42.5 KiB|10.76 ms ± 0.41%|  5.36 ms ± 1.29%|
|Backbone 1.1.0|58.7 KiB|12.41 ms ± 0.32%|  6.00 ms ± 0.55%|
|MooTools 1.4.5|156.7 KiB|72.66 ms ± 3.65%| 46.67 ms ± 3.97%|
|jQuery 1.9.1|262.1 KiB|97.10 ms ± 2.17%| 62.33 ms ± 2.72%|
|YUI 3.12.0|330.4 KiB|81.03 ms ± 1.24%| 49.21 ms ± 1.53%|
|jQuery.Mobile 1.4.2|442.2 KiB|163.28 ms ± 2.56%|105.65 ms ± 3.26%|
|Angular 1.2.5|701.7 KiB|141.96 ms ± 2.08%| 83.99 ms ± 2.02%|
|Total|1994.3 KiB|579.2ms|359.2ms|
|Change||5.41%|0.23%|