Implement PIN 6

[cicdw]

https://docs.prefect.io/guide/PINs/PIN-6-Remove-Constant-Tasks.html

Basically, to remove the auto-generation of "Constant" task and instead store constants in the flow object.

[cicdw]

@jlowin I have a different proposal for how to implement this, based on mapping over a constant list:

add_one.map(x=[1, 2, 3])

With this use case in mind, it's easier to implement a special Edge property called upstream_constant or something, which tracks whether the upstream task is a constant or not. In fact, we could compute this on the fly by checking whether the upstream is a Task instance or not. Alternatively, a new initialization kwarg which holds an upstream_constant instead of an upstream_task.

Implemented this way, the information is still contained entirely within the Flow, but it also allows for edges such as

Edge(upstream_constant=[1, 2, 3], downstream_task=add_one, key='x', mapped=True)

which handle mapping over constants.

[jlowin]

Following offline conversation in which we realized that any attempt to modify Edge objects would essentially reimplement Constant Tasks by another name, we've decided to decline PIN 6 (for now) -- See #917

[cicdw]

Closed via #915