Added support for {.packed.} pragma on objects

Added tests for packed and union pragmas

tests/objects/tobjpragma.nim

@@ -0,0 +1,49 @@
+discard """
+  file: "tobjpragma.nim"
+  output: '''2
+3
+9
+257
+1
+2
+3'''
+"""
+
+# Test 
+
+type
+  Foo {.packed.} = object
+    a: int8
+    b: int8
+
+  Bar {.packed.} = object
+    a: int8
+    b: int16   
+    
+  Daz {.packed.} = object
+    a: int32
+    b: int8 
+    c: int32  
+
+
+var f = Foo(a: 1, b: 1)
+var b: Bar
+var d: Daz
+
+echo sizeof(f)
+echo sizeof(b)
+echo sizeof(d)
+echo (cast[ptr int16](f.addr)[])
+
+type
+  Union {.union.} = object
+    a: int8
+    b: int8
+
+var u: Union
+u.a = 1
+echo u.b
+u.a = 2
+echo u.b
+u.b = 3
+echo u.a