Fix refactoring of Slot classes
Fix refactoring of Slot classes

The previous refactoring introduced a regression in that a property
could not have a Java-based getter and a JavaScript-based setter, or vice
versa. Combine both of those into a single slot type to avoid this
problem.

Also, introduce a LambdaSlot type to define a JavaScript property that
is implemented using a pair of lambda functions, and introduce a test for
that.

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