minimal example for modifying map entries.

requires both an implementation of `IClonable` for entry types and a
modified version of `om` (cursors for `IClonable`s have to implement
`ITransact`).
support keyword fields.

breaks when `read-keyword` returns nil, e.g. when the keyword field is
empty. is this a special case? or should i set the value in the map, not
in the entry?
remove unused function.