Use to_bits() instead of bits() to avoid deprecation warning.

[tomverbeure]

What are the reasons/motivation for this change?

When I compile verific.cc, I get tons of warnings like this:

frontends/verific/verific.cc:1652:16: warning: 'bits' is deprecated: Don't use direct access to the internal std::vector<State>, that's an implementation detail. [-Wdeprecated-declarations]
 1652 |                                                         initval.bits()[bit_idx] = (*ascii_initdata == '0') ? State::S0 : State::S1;
      |                                                                 ^
./kernel/rtlil.h:888:4: note: 'bits' has been explicitly marked deprecated here
  888 |         [[deprecated("Don't use direct access to the internal std::vector<State>, that's an implementation detail.")]]

When I replace bits() by to_bits(), the warning goes away.

Explain how this is achieved.

By replacing bits() by to_bits().

If applicable, please suggest to reviewers how they can test the change.

They need to run some tests with Verific enabled to make sure it works, because I compiled with my own Verific stub code instead of the real thing.

[widlarizer]

Sorry for not keeping an eye on warnings in the verific builds and thanks for reporting this. However to_bits() returns a copy of the bit vector, not a reference like bits(). I'll put up a fixed version later

You can use Const::set instead, or construct and modify std::vector<State> that you then construct a Const from, or refactor with Const::Builder which basically only exposes push_back(State). This might seem like a hassle but there's memory and runtime reasons for removing this direct bit vector access