Allow IO maps to use channels as keys

[liamhuber]

E.g. from

@macro_node()
def my_example(macro: Macro):
    macro.foo = foo()
    macro.bar = bar(i=macro.foo)
    macro.inputs_map = {"foo__widget": "thingy"}
    macro.outputs_map = {"bar__hide_it": None}

to

@macro_node()
def my_example(macro: Macro):
    macro.foo = foo()
    macro.bar = bar(i=macro.foo)
    macro.inputs_map = {foo.inputs.widget: "thingy"}
    macro.outputs_map = {bar.outputs.hide_it: None}

So that we can leverage tab-completion when defining the LHS of the pairs.

I guess implementation will involve something like adding something like

@property 
def mangled_name(self):
    return self.node.label + "__" + self.label

and then when parsing the IO maps, using key.mangled_name when isinstance(key, Channel).

[liamhuber]

Came up in the context of discussing #85

[liamhuber]

There are no more IO maps (#276 etc)