Implemented opportunity shrink failing data

Added rantly/shrinks.rb which contains the shrinking
properties of Array, String, and Integer
Fixed bug in Array shrinking

First checked if the element in the array responds to "shrinkable?"
before calling the method
Added shrinks for Hash
Add test case for 1.shrink (should shrink to 0)

1.shrink was shrinking to -1 before
add test for String#shrink
Array should shrink by length if none of its element is shrinkable
Should shrink hash by deleting an elemment if no value is shrinkable
Make shrink's monkey patching opt-in
Merge branch 'shrinkable'