symstate: merge exec_state into
normalization: separete header and implementation
treeaut: code beautification
symstate: make a derived class from LinkTree
symexec: code beautification
symexec: code beautification
backward_run: add initial attempt
RegisterAssignment: add instruction

RegisterAssignment is a base instruction for all instructions which
simply assign a value to a register.
microcode_rev: add stub implementation of reverseAndIsect()
microcode: introduce VoidInstruction

VoidInstruction is an instruction that has void effect on the symbolic
state.
microcode: change base class for certain instructions

Make certain instructions derived from RegisterAssignment
microcode: change base class for certain instuction
call: implement reverseAndIsect())
jump: add reversAndIsect() method
fixpoint: add reverseAndIsect() method
comparison: make a subclass of RegisterAssignment
abstractinstruction: add reverseAndIsect() method
backward_run: start of implementation
symexec: formating
microcode: use enqueue(state) method

Use now enqueue(state) instead of enqueue(state, next_) in order to have
more efficient copy of symbolic state.