Test that immovable types are allowed in Maybe
Test that immovable types are allowed in Maybe

Immovable types can be already wrapped into `Maybe`, which can be constructed as default `nullptr`, set to a constructed value via `.emplace()`, but couldn't be cleared back to `nullptr`. The previous commit fixed the bug, this commit adds a test.

mologie/capnproto,mologie/capnproto,mologie/capnproto