BREAKING CHANGE: Operator for dfvars is now := and not <=.
There really isn't a confusion about whether <= is to be interpreted
as the "less than or equal to" comparison or the dfvar bind.
However, := is just plain clearer as it alludes to assignment
operation in other languages. We're interpreting such an assignment
here as a lazy kind of assignment.

Minor - ReceivedChannelValue and MergedChannelValue are now one
and the same ChannelValue object.
v0.7.0