Test `FuncDefn` circuits instead of `DFG`s

[aborgna-q]

We currently parse tket1 circuits into a DFG node, and store their name in a name metadata entry.
Circuits defined in guppy use a FuncDefn instead, and store their name in the function op definition.

We should drop the metadata entry, and rely on function definitions. DFG-defined circuits will not define a name.

FuncDefns require defining the extension deltas in their definitions. This seems to cause issues with rewrite operatios, as they currently do not allow for modifying the set of extensions.