Optimize generic array length
Optimize generic array length

java.lang.reflect.Array.get_length is a jdk intrinsic.

jmh bench shows about 3x speedup
lenNew  avgt   10  13.343 ± 0.265  ns/op
lenOld  avgt   10  36.554 ± 1.057  ns/op

http://gist.github.com/isaacl/a365bf39afd81964b68f1c94f1c2b1a0/raw

scala/scala,martijnhoekstra/scala,lrytz/scala,martijnhoekstra/scala,scala/scala,martijnhoekstra/scala,scala/scala,martijnhoekstra/scala,lrytz/scala,scala/scala,martijnhoekstra/scala,scala/scala,lrytz/scala,martijnhoekstra/scala,lrytz/scala,scala/scala,lrytz/scala,lrytz/scala