Progress toward indirect JNI references.

Progress toward indirect JNI references.

Switch from simple typecasts to conversion functions for the objects
passed in and out of JNI calls.  No real change here; object references
are still just pointers.

Use explicit pin/unpin calls for primitive arrays.  For GetStringChars
we now pin the char[] rather than the String object.  (Which doesn't
make much difference in the grand scheme of things, since you need to
keep the String to pass into the release function anyway.)