feat(hydroflow_lang): initial flo implementation

[MingweiSamuel]

Basic first pass implementation, without changing the current scheduler, no breaking behavior. Adds the flo syntax (loop { ... }), basic graph structure checks, nested loop blocks, and two basic windowing operators (batch() and all_once()).

Next steps:

• Needs scheduler changes:
  • Implement un-windowing operators
  • Implement windowing operator repeat_n() -
• Pipeline flags: Needs checking of bounded vs unbounded for batch() vs all_once()
• Needs checking that all inputs into a loop agree
• State type negotiations (for batch() and handoffs (?) - don't always use Vec)
• Dag performance optimizations (lots of things)

[shadaj]

Nice! Cool that we can do this in a somewhat backwards-compatible way.