Allow imported functions in Filter, Map and Reduce

The Filter, Map and Reduce accept currently either a Lambda expression or a reference to FunctionDefinition:

( lambda_ = Lambda | function = [FunctionDefinition] )

To allow using imported external functions, this should be generalized to

( lambda_ = Lambda | function = [Function] )

or even safer (see FunctionCall rule):

( lambda_ = Lambda | function = [Function|TrueID] )

A test should be added to the test set to test Filter, Map and Reduce with an imported library function.

BTW: The current tests for Filter, Map and Reduce cover only only Lambda but do not cover the possible case of FunctionDefinition.