Add syntax configuration options.
Fix `any_instance` so it doesn't rely upon `should_receive`/`stub` being on every object.
Don't rely upon #null_object? being on every object.

This is based upon the logic in https://github.com/rspec/rspec-expectations/commit/5fbe94ae574b0daa10869675dfe0a675d268ec0b .

This is the last change we need to be able to use the :wrapped syntax with the :direct syntax disabled.