(Binary Analysis) Symbolic use-def analysis can be turned on/off
(Binary Analysis) Updated test cases for symbolic use-def analysis

* Added call to set_compute_usedef()

* Changed test answers since an SValue with an empty use-def is
  printed as if use-def was not computed.
(Util) Map.getOrElse() returns a reference type
(Binary Analysis) Thunk return edges are handled correctly
(Binary Analysis) InsnCFG return branches correct over thunks

In an instruction-based CFG, the function return branches emanating
from, e.g., x86 RET instructions, correctly point across thunks rather
than back into the thunk.  E.g.,

_start: 	call thunk
 		hlt
thunk:		jmp callee
callee:		ret

The RET instruction has a CFG edge back to the HLT.  This also
works correctly for thunks that branch to thunks, that branch to
thunks, ...., that eventually branch to a function.