Add ability to declare constants to WorkflowBuilder Operations

Inputs that will not be connected to the outputs of other workflow operations can now be declared constants.  Once declared, they will automatically be connected to the `input connector` of the DAG operations they are added to.  To ensure that name collisions do not occur the name on the `input connector` will be `<operation_name>.<input_name>`.  If an operation with any constant values is added to a DAG operation, the corresponding input on the DAG operation is also a constant (they automatically propagate).
In case it's of interest, coercing `@_` into a hash will squash duplicate keys before the below loop has a chance to see that it's being declared twice.