Re-examine argument storage in effective statements

With YANGTOOLS-1066 out of the way it becomes obvious we may be storing the argument multiple times when we are dealing with original statement.

More specifically, AbstractLeafEffectiveStatement has two specializations, storing argument in an explicit field. This statement is a CopyableNode, hence it may have an 'original' pointer – but when it does not, as is the case for EmptyLeafEffectiveStatement, the argument and declared.argument() should be pointing to the same thing. Hence we should be able to specialize a bit more.

This is probably true of many other statements as well. Audit all statements bearing a QName argument.