refactor enum code to do forward references of members

[WalterBright]

Refactored to switch to lazy evaluation of enum members, which also enables enum members to be evaluated in any order.

[WalterBright]

Once this is pulled, I intend to extend it so that enum members for imports are not semantically evaluated at all unless they are used. This is a first step to making all semantic evaluation of imports lazy, which should give us a big boost in compilation speed, as well as do a much better job at handling forward references.