Add support for multiple interfaces

example/src/main.nub

@@ -5,28 +5,34 @@ interface Printable
     func print()
 }
 
-struct Human : Printable
+interface Stringable
+{
+    func str(): cstring
+}
+
+struct Human : Printable, Stringable
 {
     name: cstring
 
+    func str(): cstring
+    {
+        return this^.name
+    }
+
     func print()
     {
-        puts(this^.name)
+        puts(this^.str())
     }
 }
 
 func main(args: []cstring): i64
 {
-    let human = alloc Human {
+    let human: Stringable = alloc Human {
         name = "oliver"
     }
 
-    human.print()
-    print_printable(human)
+    puts(human.str())
+    // human.print()
+
     return 0
 }
-
-func print_printable(printable: Printable)
-{
-    printable.print()
-}

src/compiler/NubLang/Generation/QBE/QBEGenerator.Expression.cs

@@ -459,9 +459,23 @@ public partial class QBEGenerator
     {
         var implementation = EmitUnwrap(EmitExpression(interfaceInitializer.Implementation));
 
+        var vtableOffset = 0;
+        foreach (var interfaceImplementation in interfaceInitializer.StructType.InterfaceImplementations)
+        {
+            if (interfaceImplementation == interfaceInitializer.InterfaceType)
+            {
+                break;
+            }
+
+            vtableOffset += interfaceImplementation.Functions.Count * 8;
+        }
+
         var result = TmpName();
         _writer.Indented($"{result} =l alloc8 16");
-        _writer.Indented($"storel {StructVtableName(interfaceInitializer.StructType.Name)}, {result}");
+
+        var interfaceVtablePointer = TmpName();
+        _writer.Indented($"{interfaceVtablePointer} =l add {StructVtableName(interfaceInitializer.StructType.Name)}, {vtableOffset}");
+        _writer.Indented($"storel {interfaceVtablePointer}, {result}");
 
         var objectPointer = TmpName();
         _writer.Indented($"{objectPointer} =l add {result}, 8");