- Materialize unit when its value is actually needed.
- Special-case Unit_getInstance return type at codegen. It should be a
proper Unit object instead of a "void"
Use it for char boxing/unboxing and unit materialization.
Possible to use for other purposes, for example, to add type checks
to dynamics.
See KT-18793, KT-17915, KT-19081, KT-18216, KT-12970, KT-17014,
KT-13932, KT-13930