Be able to show multiplicity of identical edges going to a net

[mdko]

Problem

When the same wire is the source to a particular net multiple times (e.g. a c net), net_graph(), and by extension all the graphical output functions that use it like output_to_graphviz(), don't properly display the additional edges.

For example, given:

import pyrtl

c = pyrtl.Const(1, 1)
o = pyrtl.Output(3, 'o')
o <<= pyrtl.concat(c, c, c)

the logic of block is:

for net in pyrtl.working_block():
    print(net)

tmp0/3W <-- c -- const_0_1/1C, const_0_1/1C, const_0_1/1C 
o/3O <-- w -- tmp0/3W 

however the result of net_graph() previously was:

for src, vals in pyrtl.net_graph().items():
    for dst, edge in vals.items():
        print(f"({src}) - {edge} -> ({dst})"

(o/3O <-- w -- tmp0/3W ) - o/3O -> (o/3O)
(tmp0/3W <-- c -- const_0_1/1C, const_0_1/1C, const_0_1/1C ) - tmp0/3W -> (o/3O <-- w -- tmp0/3W )
(const_0_1/1C) - const_0_1/1C -> (tmp0/3W <-- c -- const_0_1/1C, const_0_1/1C, const_0_1/1C )

This problem is evident in the visual output as well:

Solution

net_graph has been updated so that there are lists of edges between nodes.

for src, vals in pyrtl.net_graph().items():
    for dst, edges in vals.items():
        for edge in edges:
            print(f"({src}) - {edge} -> ({dst})")

producing

(tmp0/3W <-- c -- const_0_1/1C, const_0_1/1C, const_0_1/1C ) - tmp0/3W -> (o/3O <-- w -- tmp0/3W )
(o/3O <-- w -- tmp0/3W ) - o/3O -> (o/3O)
(const_0_1/1C) - const_0_1/1C -> (tmp0/3W <-- c -- const_0_1/1C, const_0_1/1C, const_0_1/1C )
(const_0_1/1C) - const_0_1/1C -> (tmp0/3W <-- c -- const_0_1/1C, const_0_1/1C, const_0_1/1C )
(const_0_1/1C) - const_0_1/1C -> (tmp0/3W <-- c -- const_0_1/1C, const_0_1/1C, const_0_1/1C )

It now looks like:

[mdko]

Here's another quick example:

Before

After

[mdko]

Fixes #357