Phased generator support

[ducky64]

Currently, generators only run in one stage. This makes dependencies very coarse, eg for a switching converter, the voltage won't be available until the entire generator runs, even though it could be ready when the feedback pin is solved. But the generator could depend on current, so now current can't depend on actual voltage.

There are some model-induced limitations. Mostly that links cannot expand, so connectivity must be determined in one-shot.

Ideas:

• Separate value generators and topology generators. The latter is more restrictive and one-shot, but there can be multiple of the former.
  • Or generally redefining Generators as topology generators, with some separate mechanism for values?
  • Separating topology generators also allows contents to be defined better
• General support for lambda functions (independent of generators | topology generators). Need some thought about how to handle multi-assigns.