Guppy programs cannot be loaded into tket2

[aborgna-q]

When we moved to loading packages instead of hugrs (#680) we removed the guppy-specific load_guppy_* methods. These actually were doing some hacky fixes that are currently required to load a function from guppy, so now loading such a function results in an error.

Guppy defines a CFG block inside each function definition, containing one or more DataflowBlocks with the actual circuit. In the guppy loaders we used to look inside the functions and extract the single dataflow block if possible.

The clean solution would be to finish implementing #750 and #636.
The quick-and-dirty solution is to bring back the load_guppy_* methods.

[aborgna-q]

For reference, here's the hacky guppy-function unwrapping code removed in that PR:
https://github.com/CQCL/tket2/pull/680/files#r1961704420