session.node() should return a clearer enum

[dherman]

To avoid the complicated conditionals, the session.node() method should return an enum of which "kind" of Node we got (global vs project). That enum should have a method that produces an Option<Version>.

This maybe wants to be reconciled with the Installed type returned by install_node.

[mikrostew]

FYI session.node() was renamed to session.current_node() (see a4c4a31#diff-bee44139ca738f1c26ba310a684d6ddfR62)

[charlespierce]

While we don't have a session.current_node method any more, we did recently add the ability to show where the session.current_platform() came from in #466