API is now an abstract class instead of an Interface. This is to
allow protected methods that give access to package-scoped methods
and variables inside MachineState.
MachineState now supports a different set of constants based on AT
version.
MachineState's methods and variables have had their scopes tightened
up and getters/setters added where appropriate.
MachineState.parseHeader() inlined into the constructor that calls it
so it can set public final variables.
Some reordering and additional comments in MachineState.
OpCode enum entries refactored so that:
a) variables provided via MachineState "state" are not explicitly
passed as well
b) args to each opcode are pre-fetched from the codeByteBuffer before
calling and only need to be cast before use. this comes almost
"for free" thanks to OpCodeParam.fetch
Calling functions from OpCode now cleaner as there's no write-access
to programCounter, only changing codeByteBuffer's position.
Also in OpCode, state.getProgramCounter() now provides a consistent
before-opcode position for branches, jumps, etc.
Lots of repeated code refactored out of unit tests into ExecutableTest
class. ExecutableTest also provides helper methods for examining
post-execution data values, stack positions and entries.
