The option is serialized to RTLIL as `#` (to match Python's and Rust's
option with the same symbol), and sets the `show_base` flag. Because
the flag is called `show_base` and not e.g. `alternate_format` (which
is what Python and Rust call it), in addition to the prefixes `0x`,
`0X`, `0o`, `0b`, the RTLIL option also prints the `0d` prefix.
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