Hide BeginNode/EndNode and use a finer-grained coloring scheme for effect nodes

[eregon]

Fixes #45.

examples/ruby/example_instance_of.bgv:5:
Before:

After:

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examples/ruby/example_while_break.bgv:5:
Before:

After:

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examples/graalvm-ce-java11-21.2.0/fib-ruby.bgv.gz:3:
Before:

After:

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"Empty if":
Before:

After:

[eregon]

I added svg graphs to compare before/after.

[eregon]

In mid-tier we have BeginNodes with multiple output edges:

Should we leave those in the graph?
To remove them we'd need more logic to choose which edge properties to prefer.
And also then those anchor would be on the if and not only on the Begin of the F (false) branch.
So I guess better leave them in if multiple inputs or outputs edges.

[chrisseaton]

Leave them in unless they're very simple.

[chrisseaton]

Not convinced about this purple but I'll merge it for now.

[eregon]

Not convinced about this purple but I'll merge it for now.

Feel free to tweak. I think I'll take anything except red for memory accesses, because red is already used for control flow and control flow is completely unrelated to memory accesses.

[chrisseaton]

control flow is completely unrelated to memory accesses

Both are usually fixed, that's why.