Remove function invocation counter

Remove: The function interface no longer requires functions to provide a
getInvocations() method that returns the internal invocation counter.
Remove function's getExecutor(name)

Remove: The function interface no longer requires functions to provide a
getExecutor(String name) method that returns the function executor that
is registered under the given name.
Rename GetterSupplier to ValueSupplier

Add: The old GetterSupplier interface is now called ValueSupplier.
Add ValueSupplierDefinition

Add: The ValueSupplierDefinition interface is implemented by custom
feature definition classes which define a feature that is implementing
the ValueSupplier interface (a lot of generics going on here ...).