Use underground belts when connecting machines

[peso]

Given a production goal (desired factory output, provided factory input), the blueprint generation has 3 major phases

1. Production design, which selects recipies and multipliers that reaches production goals.
2. Assembly-line design, which outlines flow at single-machine-level and rough placement
3. Transport-design, which specifies at surface level how items are moved between machines using transport-belts and inserters.

The last phase should be able to utilise underground belts.

[Buildasaurus]

Best done after #7 is implemented