Simplify rule for implicit values of enumeration members

Enumeration members have an optional Value attribute for specifying an integer equivalent to the symbolic member name that is for ordering.

The current rule for implicitly numbering the enumeration members is cloned from the C family of programming languages and allows specifying values for some members while having the other members being numbered implicitly. This can lead to surprising results for the uninitiated:

<EnumType Name="ShippingMethod">
<Member Name="VIP"/>
<Member Name="FirstClass"/>
<Member Name="TwoDay" Value="0"/>
<Member Name="Overnight"/>
</EnumType>

The result is that both VIP and TwoDay get the value zero and both FirstClass and Overnight get the value 1.

While we see value in having Value optional, we see no value in providing a value for some members only.