Refactor (split or rename) Signature

[acl-cqc]

• At the least, make Signature store an Option rather than FunctionType, so we can return a Signature for non-dataflow ops
• But, there are many things called signature (FunctionType's, Signature's, etc.), which quickly confuses as to what is what; so some renaming is in order
• Consider replacing NodeType::signature(&self) -> Signature with a simple getter for input_extensions (returning Option<ExtensionSet>); the rest of the info in the old signature() is available from op_signature() if required

[acl-cqc]

Note this substantially addressed by #700; Signature can probably now be removed altogether, following #692.