Speed up Array#split when block is passed

Rehearsal ---------------------------------------
old   8.540000   0.160000   8.700000 (  8.706186)
new   7.590000   0.100000   7.690000 (  7.689902)
----------------------------- total: 16.390000sec

          user     system      total        real
old   8.360000   0.140000   8.500000 (  8.508558)
new   7.510000   0.130000   7.640000 (  7.647456)