Add assertion preventing function references inside cursors

[dustingetz]

trafficstars

See https://github.com/dustingetz/react-cursor/pull/86 https://github.com/dustingetz/react-cursor/issues/19

[dustingetz]

Need to understand how to deal with class instances and objects with prototypes in cursor first

[newyankeecodeshop]

Disallowing the use of more complex types like Date, Map, Set might be problematic for cursor users down the road. I know there is something nice about having state that can be serialized to/from JSON or DOM storage, but not every component needs that. Something to consider.