Implement critical JNI endpoints and primitive array pass-through

[headius]

In jnr/jnr-ffi#68 we learned that there's a way to bind JNI methods with considerably reduced overhead, if you follow some rules detailed here:

http://stackoverflow.com/questions/36298111/is-it-possible-to-use-sun-misc-unsafe-to-call-c-functions-without-jni/36309652#36309652

In order to support the ability to mark bound functions as "critical" and primitive arrays as "pass through" in jnr/jnr-ffi#86, we need to implement some new native bindings in jffi and possibly ways to tell the invokes that the primitive arrays being passed are not to be dealt with in the usual JNI way.