Remove unused and confusing R0 method
Add a compile-time option to add debugging symbols

Also:
* remove the now obsolete debug-jar Ant target;
* document the compile-time options in the readme file.
Swap null check and dereferencing

Otherwise, the check is just useless.
Add a proper equals() method for Instruction

And add some unit tests for it.
Use import static for junit.Assert in CpuTests

Bytes are precious. Save them.
Fix some potential null dereferences in the FPU