JNI (Verilator) support.

Add JNITestAPI.java interface and cpp implementation.
We need to use System.load() to load the compiled (but fixed) cpp code,
but use dlopen/dlsym/dlclose in the fixed cpp code to load the shared library since Java doesn't provide an explicit unload (dlclose) mechanism.
Tease apart the pieces im sim_api.h so we can use them from both separate process ans shared library verilator implementations.
Remove unused imports.
Modify the generated verilator <Module>.mk to add rules to build shared library version of the simulator.
Use multiline string literal to compose the harness code.
Much cleanup and testing required.
TODO:
 - we hang un failed expects with some possibly JNI related semaphore.
 - break up finish/abort/unload so they're composable.