Add prototype operations.program_flow module.

vmtranslator/operations/program_flow.py
	-Add a prototype program-flow module, which implements all of
	the VM's program-flow operations (`goto`, `if-goto`, and
	`label`).
	-Define `ProgramFlowOp`, `LabelOp`, `GotoOp`, and `IfGotoOp`
	classes, with functional (and lightly tested) methods. Still
	need documentation.