Add computable declaration fields

Author: someone-with-default-username

Grammars/SimpleSharp.Compiler/ClassDeclaration.nitra

@@ -7,5 +7,7 @@ namespace JetBrains.Nitra.Samples.SimpleSharp
     Baz : int -> Nitra.NSpan * Nitra.NSpan? * Nitra.NSpan+ -> Nitra.Ast?;
     Foo : Nitra.Ast+;
     Bar : Nitra.NSpan? * Nitra.Ast;
+    Bar2 : Nitra.NSpan? * Nitra.Ast = Bar;
+    Paz : int { 4 * 2 }
   }
 }

Nitra/Nitra.Compiler/Generation/Declarations/DeclarationEmitter.n

@@ -42,7 +42,16 @@ namespace Nitra.Compiler
 
       foreach (fieldSymbol in this._declarationSymbol.Fields)
       {
-        unless (fieldSymbol.HasBody)
+        if (fieldSymbol.HasBody)
+        {
+          this._tb.DefineConditional(<[ decl:
+            public $(fieldSymbol.Name : usesite) : $(makeType(fieldSymbol))
+            {
+              get { $(fieldSymbol.Body.Expr) }
+            }
+          ]>);
+        }
+        else
         {
           this._tb.DefineConditional(<[ decl:
             public $(fieldSymbol.Name : usesite) : $(makeType(fieldSymbol));