support Protocols

[carljm]

Subtasks (these do not all need to be completed before the alpha):

• Understand Protocol-instance types as a different kind of type to other Instance types (structural types vs nominal types)
• Implement subtyping and assignability relations between different structural types, and between structural types and nominal types
• Emit errors on invalid Protocol class definitions
• Emit errors on isinstance() and issubclass() runtime checks against Protocol classes that are not decorated with @runtime_checkable
• Experiment with removing existing Type variants such as Type::AlwaysTruthy and replacing them with Protocol types

PRs:

• [red-knot] Initial tests for protocols ruff#17436
• [red-knot] Understand typing.Protocol and typing_extensions.Protocol as equivalent ruff#17446
• [red-knot] Add KnownFunction variants for is_protocol, get_protocol_members and runtime_checkable ruff#17450
• [red-knot] Fix MRO inference for protocol classes; allow inheritance from subscripted Generic[]; forbid subclassing unsubscripted Generic ruff#17452
• [red-knot] Correctly identify protocol classes ruff#17487
• [red-knot] Detect (some) invalid protocols ruff#17488
• [red-knot] Move InstanceType to its own submodule ruff#17525
• [red-knot] Add more tests for protocol members ruff#17550
• [red-knot] Emit a diagnostic if a non-protocol is passed to get_protocol_members ruff#17551
• [red-knot] Infer the members of a protocol class ruff#17556
• [red-knot] Emit diagnostics for isinstance() and issubclass() calls where a non-runtime-checkable protocol is the second argument ruff#17561
• [red-knot] Ban direct instantiations of Protocol classes ruff#17597
• [red-knot] Add more tests for protocols ruff#17603
• [red-knot] micro-optimise ClassLiteral::is_protocol ruff#17703
• [red-knot] Add a new Type::ProtocolInstance variant (take 2!) ruff#17682
• [red-knot] Ban direct instantiation of generic protocols as well as non-generic ones ruff#17741