First class scope support

[flaxed]

A common scenario in development is to have fields or types to exist depending on constants set on the environment, usually Release and Debug environments.

This proposal is for a scope keyword, to work as a generic form of first class support for this case.

The scope will have the format of:

scope $(ConditionList)
{
    $(Contents)
}

The ConditionList is a comma separated list of Condition expressions. Each Condition will be a string Tag with an optional ! to negate it. Will other operations be useful?
Example:

scope (Network, !Debug)
{
}

To ease iteration on the files, a empty list, or maybe even skipping the list at all will be supported. In this case it will be as if there is no scope at all. Example: scope () { } or scope { }

The Contents is either one or more struct definitions, or one or more field definitions.
Ommiting the { } scope block delimiters for single items can be considered, example:

scope (Network, !Debug)
def struct Foo
{
}

But this feature is likely to increase parsing complexities.