Fields are the data that are sent as part of an event.

Events are just maps, where the keys are either keywords or strings and the
values may be anything. Since clj-honeycomb wraps the Java Honeycomb SDK, we
must transform some of the values from Clojure types into more Java-friendly
types where possible.

Only the ->ValueSupplier function in this namespace should be considered part
of the public API for clj-honeycomb.