Resolve self, instance, and class from the receiver in runtime type checks

[ksss]

Fixes #3192.

RBS::Test::TypeCheck received the classes for self, instance, and class as three separate arguments, and its callers filled them in inconsistently. As a result, a method called on a subclass could not be checked precisely, and rbs test did not check instance at all.

At runtime, all three are determined by the receiver's class and whether the call is an instance or a singleton method call. This PR models that as a single value:

TypeCheck.new(context: TypeCheck::InstanceContext.of(receiver), builder:, sample_size:, unchecked_classes:)
TypeCheck.new(context: TypeCheck::SingletonContext.of(receiver), builder:, sample_size:, unchecked_classes:)

	self	instance	class
InstanceContext.of(obj)	obj.class	obj.class	obj.class.singleton_class
SingletonContext.of(mod)	mod.singleton_class	mod	mod.singleton_class

Both rbs test and RBS::UnitTest::TypeAssertions now build the context from the actual receiver. In rbs test, a module's instance method called on a class that extends the module is checked as a singleton method call, as Steep does.

The subject of the errors is given separately as owner:. rbs test passes the hooked class, so error tags stay the same as before. With the reproduction from the issue, rbs test now reports all four violations:

[Foo.make] ReturnTypeError: expected `instance` but returns `#<Foo>`
[Foo#copy] ReturnTypeError: expected `self` but returns `#<Foo>`
[Foo.bad_instance] ReturnTypeError: expected `instance` but returns `1`
[Foo#bad_self] ReturnTypeError: expected `self` but returns `1`

Compatibility

• self_class:, instance_class:, and class_class: are still accepted, with a deprecation warning shown once per process. Passing them together with context: raises ArgumentError.
• Without a context, checking self, instance, or class raises TypeCheck::NoReceiverContextError. assert_type and assert_const_type use this instead of placeholder values.
• TypeAssertions#instance_class and #class_class are removed.

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