add union by default

[furesoft]

pls add a c-like union macro by default.

sample:

union color {
      r : int;
      g: int;
      value: long;
}

will produce a struct with structlayout and struct and fieldoffset

[NN---]

Current Nemerle implementation doesn't allow to make namespace level macro.
It means that supporting unions requires compiler change.
It is possible to do it using macro-attribute such as:

[Union]
struct color { r : int; g : int; value : long; }

For example 'Dto' macro converts all fields to be public and creates a constructor using 'Record' macroattribute:

// Default visibility is private, changed to public by macro
[Dto] class Task { Name : string; IsDone : bool }  

def t = Task("A", true);

https://github.com/NemerleWeb/NemerleWeb/blob/master/NemerleWeb.Macros/Data/Dto.n