Remove nix and signal-hook dependencies

[malbarbo]

This reduces the dependencies from 9 to 2, the binary size (striped) from 515kb to 435kb (15%) and the build time from 9.36s to 2.96s , while it increases the line count from 1006 to 1012.

The editor in single thread, so I think the signal handling is ok, but it need to be reviewed.

[ilai-deutel]

Thanks a lot for this!
I will try to review this PR soon. In the meantime, would you mind adding documentation for the functions you added, especially register_winsize_change_signal_handler and set_windows_size_was_changed?

[malbarbo]

I will work on that.

[ilai-deutel]

Great work!

Could please update the "Dependencies" sections of the README as well:

• Removing mentions of nix and signal-hook
• Removing the sentence "Safe wrappers are preferred...", which doesn't really apply anymore

[ilai-deutel]

Thanks!