The capnpc-java binary should be installed in libexec/ and not bin bin/

[Flowdalic]

The capnpc-java binary should be installed in libexec/ and not bin bin/, since it appears that this binary should not be invoked by the user, but only by the Cap'n Proto compiler. But please correct me if I am wrong.
I think this requires also changes to the Cap' Proto compiler, to look in libexec/ for binaries.

[dwrensha]

capnproto-java is following the lead of capnproto-c++ here: capnpc-c++ gets installed in bin/.

I'm not enough of an expert on filesystem conventions to make a judgement on whether that's the best place for it.

My preferred long term solution is for capnpc-java to be re-written in java (#111) and therefore perhaps not need to be installed system-wide (because Maven would be able to easily deal with it).