Verilog renaming uses "_", works on whole AST

Summary of changes to firrtl.passes.VerilogRename:
  - Use "_" to mangle names that conflict with Verilog
    keywords (previously "$")
  - Rewrite to operate on the whole AST to propogate mangled ports and
    module names
  - Make VerilogRename a Transform (was previously a Pass)
  - Renames are now propagated
  - Adds documentation for new VerilogRename

This makes the VerilogRename Transform (previously a Pass) use an
underscore ('_') instead of a dollar sign ('$') to mangle names that
conflict with Verilog keywords. This prevents problems with potentially
buggy tools that are not expecting '$' in Verilog names.

This reimplements VerilogRename to be safe for name collisions that may
occur anywhere in the AST, e.g., in ports, module names, circuit names, or
in any statements/expressions. Previously, names were only mangled in
statements and in place. This resulted in problems where renames of ports
in a child's namespace would not be guaranteed to be mangled the same way
in a parent's namespace. The algorithm is reimplemented to walk all
modules in reverse topological order (from leafs to main) and relying on a
RenameMap to track name changes.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>