[topgen] Naming convention for negative signals

Nobody has claimed this yet.

Topgen currently does not follow the naming guideline for low active signals. It is not possible to define a signal which gets rendered as `signal_ni` (same for outputs). If a signal is named `signal_n` it will be rendered as `signal_n_i`.

This violates our styleguide.

The only use currently is in rstmgr and ast. We should clean this up, probably by adding a polarity tag for IOs or just assume a signal name ending on `_n` is low active.

Start with topgen's signal and output naming logic, then inspect the current uses in rstmgr and ast mentioned in the issue. Decide how low-active polarity should be represented, and consider both the explicit polarity-tag and `_n` naming approaches before checking that signals such as `signal_n` render as `signal_ni` without unwanted separators.

Written by the indexing model from the issue text.