Improved specs on Range#initialize.
Added benchmark on Range#step.
Improved readability and performance of Range#step.

Before:
  (0..10000).step(1)     3848.3 (±3.3%) i/s -      19437 in   5.056733s
(0..10000).step(100)   370691.0 (±3.6%) i/s -    1849620 in   4.996711s

After:
  (0..10000).step(1)     4457.2 (±5.2%) i/s -      22506 in   5.065479s
(0..10000).step(100)   395559.5 (±8.0%) i/s -    1975052 in   5.036592s
Improved readability of Range#to_a.