Add option for a custom flatten block separator char

[mikesinouye]

What are the reasons/motivation for this change?
Enables elaborated instance name parity with the default behavior of a commercial synthesis tool.

Explain how this is achieved.
New option in flatten for a custom block separator string.

If applicable, please suggest to reviewers how they can test the change.
flatten -separator /

[mikesinouye]

Hey @povik, whenever you get a chance if you could weigh in on this change, thank you!

[povik]

I wonder if there are other places we should be offering this option for consistency, but I can't think of anything atm

[gadfort]

@povik the synth command can take in -flatten I think it would be helpful if we could set it there too

[povik]

@gadfort We would prefer a scratchpad variable for that (there are many synth passes and scratchpad lets us expose it everywhere)

[gadfort]

@povik I didn't know that was an option, I don't see that in the docs, how do you use it?

[povik]

The way you would use it is you would have

scratchpad -set flatten.separator /

or similar at the top of your script.

Then we need code inside flatten which would pick this up.