[tools,topgen] DRAFT OF Simplify topgen

[matutem]

THIS IS A DRAFT

This is a big change in topgen. Some salient changes:

• It always regenerates from scratch the ipgen in-memory configurations
  (as IpBlock objects) so it is truly incremental: the previous flow
  reused the pre-existing generated ipgen hjson if found.
• It delays the actual ipgen file generation until the complete top config
  is generated. This is a step towards breaking topgen into smaller and
  mutually independent steps, thus enabling parallelizing them.
• It converges with a single pass of process_top.

In order to accomplish this some important changes are made:

• Call some of the merge_top functions suitable for each specific ipgen
  prior to generating the in-memory hjson config. This means not all
  IpBlock objects are available, so these merge functions are instrumented
  to skip blocks missing.
• The generation of ipgens are ordered according to dependencies. This
  order created from the expected block functionality, so is not derived
  from a constructed graph, but converging in one pass means it is correct.
• Calling merge functions early means some of the objects in the in-memory
  top config are not plain dictionaries but are classes, so the code needs
  to handle either.