ipaddr: faster Equal and Exclude

Seems like complier stuff on Go 1.2 didn't have a luck.
This CL just re-enables "commit e204e76e77100de416070b7d1854dd767c4f813d"
that removes unnecessary type assertions in both Equal and Exlucde methods.

benchmark                     old ns/op    new ns/op    delta
BenchmarkIPv4Contains                30           30   +0.32%
BenchmarkIPv4Overlaps                25           25   -1.95%
BenchmarkIPv4Equal                   20           14  -30.00%
BenchmarkIPv4Subnets                482          483   +0.21%
BenchmarkIPv4Exclude               2278         2137   -6.19%
BenchmarkIPv4ComparePrefix           30           30   -1.63%
BenchmarkIPv4SummaryPrefix          137          132   -3.65%
BenchmarkIPv6Contains                25           25   +0.79%
BenchmarkIPv6Overlaps                39           41   +5.08%
BenchmarkIPv6Equal                   20           14  -27.23%
BenchmarkIPv6Subnets                645          651   +0.93%
BenchmarkIPv6Exclude              10881         9976   -8.32%
BenchmarkIPv6ComparePrefix           29           30   +0.67%
BenchmarkIPv6SummaryPrefix          199          197   -1.01%