Do not require label for singleton group key

[julianhyde]

Currently we require that each group key (expression following a group keyword in a from expression) either has an explicit label (defined using id =) or is such that an implicit label can be derived.

But if the group has only one expression and is the last step in a from then no label is needed, therefore we should not require a label.

The following expression currently throws IllegalArgumentException: cannot derive label for expression x + y but should be valid:

from p in [(1, 2), (3, 4)] group x + y

The following expression is invalid, and should remain invalid, because group not the last step (it is followed by where):

from p in [(1, 2), (3, 4)] group x + y where false