[feature quest] obfuscation should keep top level

[facebreeze]

Hi,
verilog code obfuscation is widely adopted during IP delivery in the industry, and verible-verilog-obfuscate works well on code obfuscation. However, it should gives more fine control to make it more flexible.

1. Obfuscate should keep top level module name and port name by default to allow following compile/sim flow run smoothly.
2. Some vital signals like clock/reset should be preserved in case some workflow depend on these name.
3. give control on preserve instance name and module name to make it easier during backend P&R flow.