Our "callable type" should be able to represent a callable of arbitrary signature; typing.Callable can only spell a limited subset of those types.

This also includes implementing subtyping/assignability/equivalence for this type.

• Add a new CallableType variant ([red-knot] Understand typing.Callable #16493)
• Use the new variant for typing.Callable ([red-knot] Understand typing.Callable #16493)
• Use the new variant for lambda expressions ([red-knot] Infer lambda expression #16547)
  • Infer the return type of lambda expressions ([red-knot] Infer lambda return type as Unknown #16695)
• Implement support for calling a typing.Callable i.e., call method ([red-knot] Support calling a typing.Callable #16888)
• Implement various relationship methods for the new variant
  • is_assignable_to ([red-knot] Check assignability for two callable types #16845)
  • is_gradual_equivalent_to ([red-knot] Check gradual equivalence between callable types #16634)
  • is_subtype_of ([red-knot] Check subtype relation between callable types #16804)
  • is_equivalent_to ([red-knot] Check whether two callable types are equivalent #16698)
  • is_disjoint_form ([red-knot] Disjointness for callable types #17094)
• Other methods
  • is_fully_static ([red-knot] Check if callable type is fully static #16633)
  • is_single_valued ([red-knot] Understand typing.Callable #16493)
  • is_singleton ([red-knot] Understand typing.Callable #16493)
  • member_lookup_with_policy ([red-knot] Callable member lookup, meta type impl #16618)
  • static_member ([red-knot] Callable member lookup, meta type impl #16618)
  • to_meta_type ([red-knot] Callable member lookup, meta type impl #16618)
• Add property tests for relation methods between callable types ([red-knot] Add property tests for callable types #17006)