This allows to add new kinds of keys that need more data without making
KeyValue's footprint bigger for common keys.

This changes the [_symbol] field into [_payload], which holds the same
as the previous field for more common keys but can hold bigger objects
for keys of the new "Complex" kind.

This also adds a complex key: String keys with a symbol different than
the outputted string.

Unit tests are added as the Java language is not helpful in making
robust code.