tracking issue: Improve hugr/guppy → pytket support

[aborgna-q]

Continuation from #377.
These are tasks for flattening hugrs generated by guppy, so they can be converted to pytket.
See #503 for the pytket → hugr tracking issue.

Discussion describing the problem and algorithm: #747

Related tracking issues:

• tracking issue: Control flow rewrites / flattening #749

External issues:

• Convert Conditional/TailLoop-shaped CFGs into dataflow hugr#818

[aborgna-q]

The main tasks of this tracking issue have been resolved. Closing